ci: Convert runner select to composite action (#39270)

to run bencher builds on self-hosted runners (#39269), we will need to
do a self-hosted runner select, but that would exceed the workflow call
depth limit (try → dispatch-workflow → linux → bencher →
self-hosted-runner-select).

this patch converts the self-hosted runner select from a [reusable
workflow](https://docs.github.com/en/actions/concepts/workflows-and-actions/reusable-workflows)
to a [composite
action](https://docs.github.com/en/actions/tutorials/create-actions/create-a-composite-action),
much like #38503 did for the runner timeout.

Testing:
- self-hosted
<https://github.com/servo/servo/actions/runs/17674919359/job/50235098986>
- GitHub-hosted
<https://github.com/delan/servo/actions/runs/17674928875/job/50234708810>

Fixes: part of #39269

---------

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

.github/workflows/linux.yml

@@ -99,15 +99,32 @@ jobs:
   # Runs the underlying job (“workload”) on a self-hosted runner if available,
   # with the help of a `runner-select` job and a `runner-timeout` job.
   runner-select:
-    uses: ./.github/workflows/self-hosted-runner-select.yml
-    secrets: inherit
-    with:
-      # Before updating the GH action runner image for the nightly job, ensure
-      # that the system has a glibc version that is compatible with the one
-      # used by the wpt.fyi runners.
-      github-hosted-runner-label: ubuntu-22.04
-      self-hosted-image-name: servo-ubuntu2204
-      force-github-hosted-runner: ${{ inputs.upload || inputs.force-github-hosted-runner }}
+    runs-on: ubuntu-22.04
+    outputs:
+      unique-id: ${{ steps.select.outputs.unique-id }}
+      selected-runner-label: ${{ steps.select.outputs.selected-runner-label }}
+      is-self-hosted: ${{ steps.select.outputs.is-self-hosted }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          sparse-checkout: '.github'
+      - name: Runner select
+        id: select
+        uses: ./.github/actions/runner-select
+        with:
+          monitor-api-token: ${{ secrets.SERVO_CI_MONITOR_API_TOKEN }}
+          # Before updating the GH action runner image for the nightly job, ensure
+          # that the system has a glibc version that is compatible with the one
+          # used by the wpt.fyi runners.
+          github-hosted-runner-label: ubuntu-22.04
+          self-hosted-image-name: servo-ubuntu2204
+          # You can disable self-hosted runners globally by creating a repository variable named
+          # NO_SELF_HOSTED_RUNNERS with any non-empty value.
+          # <https://github.com/servo/servo/settings/variables/actions>
+          NO_SELF_HOSTED_RUNNERS: ${{ vars.NO_SELF_HOSTED_RUNNERS }}
+          # Any other boolean conditions that disable self-hosted runners go here.
+          force-github-hosted-runner: ${{ inputs.upload || inputs.force-github-hosted-runner }}
   runner-timeout:
     needs:
       - runner-select