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(

.github/scripts/v8_canary_changes.py

@@ -21,6 +21,7 @@ ROOT = Path(__file__).resolve().parents[2]
 CANARY_PATH_PATTERNS = {
     ".bazelrc",
     ".github/actions/setup-bazel-ci/**",
+    ".github/actions/setup-ci/**",
     ".github/scripts/run_bazel_with_buildbuddy.py",
     ".github/scripts/rusty_v8_bazel.py",
     ".github/scripts/rusty_v8_module_bazel.py",
@@ -40,6 +41,7 @@ CANARY_PATH_PATTERNS = {
 # Windows source builds are a narrower, more expensive subset of the canary.
 # A V8 version change also requires them even when no path below changed.
 WINDOWS_SOURCE_BUILD_PATHS = {
+    ".github/actions/setup-ci/**",
     ".github/scripts/rusty_v8_bazel.py",
     ".github/scripts/rusty_v8_module_bazel.py",
     ".github/scripts/v8_canary_changes.py",
@@ -72,6 +74,15 @@ def canary_required(
     )
 
 
+def matching_windows_source_paths(changed_files: set[str]) -> set[str]:
+    """Return changed paths that require Windows rusty_v8 source builds."""
+    return {
+        path
+        for path in changed_files
+        if any(fnmatchcase(path, pattern) for pattern in WINDOWS_SOURCE_BUILD_PATHS)
+    }
+
+
 def resolved_v8_version(cargo_lock: bytes) -> str:
     versions = sorted(
         {
@@ -96,7 +107,7 @@ def windows_source_required(
     return (
         force
         or base_v8_version != head_v8_version
-        or not changed_files.isdisjoint(WINDOWS_SOURCE_BUILD_PATHS)
+        or bool(matching_windows_source_paths(changed_files))
     )
 
 
@@ -165,7 +176,7 @@ def main() -> None:
                 if matched_canary_paths
                 else "no relevant changes"
             )
-            matched_windows_paths = sorted(files & WINDOWS_SOURCE_BUILD_PATHS)
+            matched_windows_paths = sorted(matching_windows_source_paths(files))
             windows_source_reason = (
                 ", ".join(matched_windows_paths)
                 if matched_windows_paths