ci: share common workflow setup (#31318)

## Why

CI jobs repeat common bootstrap steps, which makes it harder to keep
Bazel and Cargo lanes aligned. Centralizing the lightweight setup gives
us one place for future runner-wide optimizations without adding Rust
toolchain or component installation to the Windows Bazel long poles.

## What

- add zero-input `.github/actions/setup-ci` to set Cargo's git
transport, install DotSlash and `just`, expose DotSlash from stable PATH
locations, and enable Windows Git long paths
- have `setup-bazel-ci` compose the common setup, then remove its
DotSlash/test-prerequisite plumbing
- migrate Bazel, Cargo CI, V8 canary, repo checks, nextest, and Windows
release call sites while keeping Rust toolchain and MSVC setup explicit
- preserve the existing nextest Dev Drive setup unchanged

## Validation

- `just test-github-scripts` (30 tests)
- parsed workflow and composite-action YAML with `yq`

.github/scripts/test_v8_canary_changes.py

@@ -4,6 +4,7 @@ import unittest
 from pathlib import Path
 
 from v8_canary_changes import changed_files
+from v8_canary_changes import canary_required
 from v8_canary_changes import merge_base
 from v8_canary_changes import resolved_v8_version
 from v8_canary_changes import windows_source_required
@@ -46,6 +47,14 @@ version = "149.2.0"
             )
         )
 
+    def test_setup_ci_change_requires_canary_and_source_build(self) -> None:
+        changed_files = {".github/actions/setup-ci/action.yml"}
+
+        self.assertTrue(canary_required(changed_files, "149.2.0", "149.2.0"))
+        self.assertTrue(
+            windows_source_required(changed_files, "149.2.0", "149.2.0")
+        )
+
     def test_manual_dispatch_requires_source_build(self) -> None:
         self.assertTrue(
             windows_source_required(