ci: Extract fast checkout steps into composite action (#40234)

this patch extracts the “faster checkout” steps we use for jobs that may
run on self-hosted runners into a composite action. the action is stored
in [servo/ci-runners](https://github.com/servo/ci-runners), to avoid an
extra checkout step, and the action calls are locked to a commit, to
avoid problems when making breaking changes.

while [servo/actions](https://github.com/servo/actions) is a reasonable
place to store this action, i’m planning to document our self-hosted CI
system and potentially make it usable by other projects, and this action
is effectively part of that system. that said, actions in that repo will
undergo
[review](00deefd2be)
as if they were in this repo.

Testing:
- self-hosted:
[windows](https://github.com/servo/servo/actions/runs/19125076579/job/54653625055#step:2:25),
[lint](https://github.com/servo/servo/actions/runs/19125076579/job/54653630737#step:2:21)
- GitHub-hosted:
[windows](https://github.com/delan/servo/actions/runs/19125003933/job/54653410264#step:2:25),
[lint](https://github.com/delan/servo/actions/runs/19125003933/job/54653410010#step:2:21)

---------

Signed-off-by: Delan Azabani <dazabani@igalia.com>

.github/workflows/linux.yml

@@ -152,26 +152,7 @@ jobs:
     name: Linux Build [${{ needs.runner-select.outputs.unique-id }}]
     runs-on: ${{ needs.runner-select.outputs.selected-runner-label }}
     steps:
-      - uses: actions/checkout@v4
-        if: ${{ runner.environment != 'self-hosted' && github.event_name != 'pull_request_target' }}
-      # This is necessary to checkout the pull request if this run was triggered via a
-      # `pull_request_target` event.
-      - uses: actions/checkout@v4
-        if: ${{ runner.environment != 'self-hosted' && github.event_name == 'pull_request_target' }}
-        with:
-          ref: ${{ github.event.pull_request.head.sha }}
-
-      # Faster checkout for self-hosted runner that uses prebaked repo.
-      - if: ${{ runner.environment == 'self-hosted' && github.event_name != 'pull_request_target' }}
-        run: git fetch --depth=1 origin $GITHUB_SHA
-      - if: ${{ runner.environment == 'self-hosted' && github.event_name == 'pull_request_target' }}
-        run: git fetch --depth=1 origin ${{ github.event.pull_request.head.sha }}
-      - if: ${{ runner.environment == 'self-hosted' }}
-        # Same as `git switch --detach FETCH_HEAD`, but fixes up dirty working
-        # trees, in case the runner image was baked with a dirty working tree.
-        run: |
-          git switch --detach
-          git reset --hard FETCH_HEAD
+      - uses: servo/ci-runners/actions/checkout@0b9edd49381b22c14e5b123f21377269864ddf54
       - name: Free Disk Space (Ubuntu)
         uses: jlumbroso/free-disk-space@main
         if: ${{ runner.environment != 'self-hosted' }}
@@ -362,25 +343,7 @@ jobs:
     runs-on: ${{ needs.runner-select-coverage.outputs.selected-runner-label }}
     continue-on-error: true
     steps:
-      - uses: actions/checkout@v4
-        if: ${{ runner.environment != 'self-hosted' && github.event_name != 'pull_request_target' }}
-      # This is necessary to checkout the pull request if this run was triggered via a
-      # `pull_request_target` event.
-      - uses: actions/checkout@v4
-        if: ${{ runner.environment != 'self-hosted' && github.event_name == 'pull_request_target' }}
-        with:
-          ref: ${{ github.event.pull_request.head.sha }}
-      # Faster checkout for self-hosted runner that uses prebaked repo.
-      - if: ${{ runner.environment == 'self-hosted' && github.event_name != 'pull_request_target' }}
-        run: git fetch --depth=1 origin $GITHUB_SHA
-      - if: ${{ runner.environment == 'self-hosted' && github.event_name == 'pull_request_target' }}
-        run: git fetch --depth=1 origin ${{ github.event.pull_request.head.sha }}
-      - if: ${{ runner.environment == 'self-hosted' }}
-        # Same as `git switch --detach FETCH_HEAD`, but fixes up dirty working
-        # trees, in case the runner image was baked with a dirty working tree.
-        run: |
-          git switch --detach
-          git reset --hard FETCH_HEAD
+      - uses: servo/ci-runners/actions/checkout@0b9edd49381b22c14e5b123f21377269864ddf54
       - name: Free Disk Space (Ubuntu)
         uses: jlumbroso/free-disk-space@main
         if: ${{ runner.environment != 'self-hosted' }}