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/actions/runner-select/action.yml

@@ -0,0 +1,100 @@
+name: Select Self-hosted Runner
+inputs:
+  monitor-api-token:
+    required: true
+    type: string
+  github-hosted-runner-label:
+    required: true
+    type: string
+  self-hosted-image-name:
+    required: true
+    type: string
+  self-hosted-runner-scope:
+    required: false
+    type: string
+    default: /orgs/${{ github.repository_owner }}/actions/runners
+  force-github-hosted-runner:
+    required: false
+    type: boolean
+    default: false
+  NO_SELF_HOSTED_RUNNERS:
+    required: false
+    type: string
+    default: ""
+outputs:
+  unique-id:
+    value: ${{ steps.select.outputs.unique_id }}
+  selected-runner-label:
+    value: ${{ steps.select.outputs.selected_runner_label }}
+  is-self-hosted:
+    value: ${{ steps.select.outputs.is_self_hosted }}
+
+runs:
+  using: "composite"
+  # Selects a self-hosted runner if available, or else a GitHub-hosted runner.
+  # We generate a unique id for the workload, then ask our monitor API to
+  # reserve a self-hosted runner for us.
+  steps:
+    - name: Select and reserve best available runner
+      id: select
+      shell: bash
+      run: |
+        github_hosted_runner_label='${{ inputs.github-hosted-runner-label }}'
+        self_hosted_image_name='${{ inputs.self-hosted-image-name }}'
+        self_hosted_runner_scope='${{ inputs.self-hosted-runner-scope }}'
+
+        set -euo pipefail
+
+        fall_back_to_github_hosted() {
+          echo 'Falling back to GitHub-hosted runner'
+          echo "selected_runner_label=$github_hosted_runner_label" | tee -a $GITHUB_OUTPUT
+          echo 'is_self_hosted=false' | tee -a $GITHUB_OUTPUT
+          exit 0
+        }
+
+        # Generate a unique id that allows the workload job to find the runner
+        # we are reserving for it (via runner labels), and allows the timeout
+        # job to find the workload job run (via the job’s friendly name), even
+        # if there are multiple instances in the workflow call tree.
+        unique_id=$(uuidgen)
+        echo "unique_id=$unique_id" | tee -a $GITHUB_OUTPUT
+
+        # Disable self-hosted runners by creating a repository variable named
+        # NO_SELF_HOSTED_RUNNERS with any non-empty value.
+        # <https://github.com/servo/servo/settings/variables/actions>
+        if [ -n '${{ inputs.NO_SELF_HOSTED_RUNNERS }}' ]; then
+          echo 'NO_SELF_HOSTED_RUNNERS is set!'
+          fall_back_to_github_hosted
+        fi
+
+        if [ '${{ inputs.force-github-hosted-runner }}' = true ]; then
+          echo 'inputs.force-github-hosted-runner is set!'
+          fall_back_to_github_hosted
+        fi
+
+        for monitor_api_base_url in $(printf \%s\\n \
+          https://ci0.servo.org \
+          https://ci1.servo.org \
+          https://ci2.servo.org \
+        | shuf); do
+          # Use the monitor API to reserve a runner. If we get an object with
+          # runner details, we succeeded. If we get null, we failed.
+          take_runner_url=$monitor_api_base_url/profile/$self_hosted_image_name/take\?unique_id=$unique_id\&qualified_repo=${{ github.repository }}\&run_id=${{ github.run_id }}
+          result=$(mktemp)
+          echo
+          echo POST "$take_runner_url"
+          if curl -sS --fail-with-body --connect-timeout 5 --max-time 30 -X POST "$take_runner_url" \
+              -H 'Authorization: Bearer ${{ inputs.monitor-api-token }}' > $result \
+              && jq -e . $result > /dev/null; then
+            echo
+            echo "selected_runner_label=reserved-for:$unique_id" | tee -a $GITHUB_OUTPUT
+            echo 'is_self_hosted=true' | tee -a $GITHUB_OUTPUT
+            exit 0
+          fi
+        done
+
+        cat $result
+        echo
+        echo
+        echo 'No self-hosted runners available!'
+        fall_back_to_github_hosted