ci: route build IO through Dev Drives (#31357)

## Why

Windows Cargo and Bazel jobs spend significant time in filesystem-heavy
build and cache directories. Route those directories through one CI
build root so Windows can use its Dev Drive and Unix can use a stable
cache root.

## What

- Have `setup-ci` define `CI_BUILD_ROOT`, `CARGO_TARGET_DIR`, Bazel
cache/output paths, and temp paths.
- Require Windows to find or provision a verified Dev Drive instead of
falling back to `C:`.
- Pass the shared Bazel output base to `setup-bazel` so its explicit
`output_base` does not defeat Dev Drive routing.
- Point nextest, release, and V8 source-build paths at the shared
environment contract.

## Benchmark results

One-off cold-cache WPR/ETW traces show the explicit Bazel output-base
routing removes the dominant `C:` traffic:

| sample | `C:\_bazel` | summed `C:` traffic | traced test step |
|---|---:|---:|---:|
| shard 1 before | 62.2 GiB | 85.2 GiB | 16m22s |
| shard 1 updated | 0 | 16.5 GiB | 12m05s |
| shard 3 before | 67.2 GiB | 84.6 GiB | 16m48s |
| shard 3 updated | 0 | 13.5 GiB | 11m08s |

For a cold x64 V8 source build, the retained build-tail sample showed
`D:\cargo-target` at ~1.29 GiB while measured `C:` roots totaled ~0.45
GiB (`C:\Users` ~0.33 GiB, `C:\Program Files` ~0.06 GiB, `C:\Windows`
~0.03 GiB). The full cold build took 2h20m36s.

The Bazel timing improvement is directional because both refreshed
shards failed tests. The V8 trace is a bounded build-tail sample, not
the full build. All final samples had zero lost ETW events; VHDX traffic
was excluded from the optimization ranking.

Runs: [baseline
Bazel](https://github.com/openai/codex/actions/runs/28911908527),
[updated
Bazel](https://github.com/openai/codex/actions/runs/28917133701), [V8
build tail](https://github.com/openai/codex/actions/runs/28933626678).

## Manual validation

- Ran `just fmt`.
- Ran `just test-github-scripts` (35 tests).
- Parsed GitHub Actions YAML with `yq`.
- Ran `git diff --check`.

## Stack

- [#31332](https://github.com/openai/codex/pull/31332) — parameterize
Cargo target paths
- [#31356](https://github.com/openai/codex/pull/31356) — Windows 2025
runner bump
- [#31357](https://github.com/openai/codex/pull/31357) — Dev Drive I/O
routing

.github/workflows/rust-ci-full-nextest-platform.yml

@@ -71,11 +71,6 @@ jobs:
         with:
           persist-credentials: false
 
-      - name: Configure Dev Drive (Windows)
-        if: ${{ runner.os == 'Windows' }}
-        shell: pwsh
-        run: ../.github/scripts/setup-dev-drive.ps1
-
       - uses: ./.github/actions/setup-ci
 
       - name: Install Linux build dependencies
@@ -136,11 +131,7 @@ jobs:
             echo "Using sccache GitHub backend"
           else
             echo "SCCACHE_GHA_ENABLED=false" >> "$GITHUB_ENV"
-            if [[ -n "${DEV_DRIVE:-}" ]]; then
-              echo "SCCACHE_DIR=${DEV_DRIVE}\\.sccache" >> "$GITHUB_ENV"
-            else
-              echo "SCCACHE_DIR=${{ github.workspace }}/.sccache" >> "$GITHUB_ENV"
-            fi
+            echo "SCCACHE_DIR=${CI_BUILD_ROOT}/sccache" >> "$GITHUB_ENV"
             echo "Using sccache local disk + actions/cache fallback"
           fi
 

.github/workflows/sdk.yml

@@ -160,8 +160,7 @@ jobs:
         continue-on-error: true
         uses: actions/cache/save@668228422ae6a00e4ad889ee87cd7109ec5666a7 # v5.0.4
         with:
-          path: |
-            ~/.cache/bazel-repo-cache
+          path: ${{ steps.setup_bazel.outputs.repository-cache-path }}
           key: bazel-cache-x86_64-unknown-linux-gnu-${{ hashFiles('MODULE.bazel', 'codex-rs/Cargo.lock', 'codex-rs/Cargo.toml') }}
 
       - name: Check for a clean worktree

.github/workflows/v8-canary.yml

@@ -308,8 +308,6 @@ jobs:
     runs-on: ${{ matrix.runner }}
     permissions:
       contents: read
-    env:
-      CARGO_TARGET_DIR: ${{ github.workspace }}/upstream-rusty-v8/target
     strategy:
       fail-fast: false
       matrix:
@@ -322,7 +320,8 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
 
-      - uses: ./.github/actions/setup-ci
+      - id: setup_ci
+        uses: ./.github/actions/setup-ci
 
       - name: Configure git for upstream checkout
         shell: bash
@@ -366,8 +365,8 @@ jobs:
         uses: actions/cache@668228422ae6a00e4ad889ee87cd7109ec5666a7 # v5
         with:
           path: |
-            ${{ env.CARGO_TARGET_DIR }}/sccache
-            ${{ env.CARGO_TARGET_DIR }}/${{ matrix.target }}/release/gn_out
+            ${{ steps.setup_ci.outputs.cargo-target-dir }}/sccache
+            ${{ steps.setup_ci.outputs.cargo-target-dir }}/${{ matrix.target }}/release/gn_out
           key: rusty-v8-source-${{ matrix.target }}-${{ matrix.runner }}-sandbox-${{ hashFiles('upstream-rusty-v8/Cargo.lock', 'upstream-rusty-v8/build.rs', 'upstream-rusty-v8/git_submodule_status.txt') }}
           restore-keys: |
             rusty-v8-source-${{ matrix.target }}-${{ matrix.runner }}-sandbox-
@@ -376,7 +375,7 @@ jobs:
         shell: pwsh
         env:
           SCCACHE_CACHE_SIZE: 256M
-          SCCACHE_DIR: ${{ env.CARGO_TARGET_DIR }}/sccache
+          SCCACHE_DIR: ${{ steps.setup_ci.outputs.cargo-target-dir }}/sccache
           SCCACHE_IDLE_TIMEOUT: 0
         run: |
           $version = "v0.8.2"