ci: Move runner-timeout to composite action (#38503)

We are hitting the limit of 20 workflow references in servo. To help
mitigate this somewhat, we migrate the timeout job to a composite
action, which should be able to support our needs just as well.

Testing: [try run of this
PR](https://github.com/servo/servo/actions/runs/16783916103/job/47529580725).
[try run with unconditional
cancel](https://github.com/servo/servo/actions/runs/16784074213/job/47530099654).

This reduces our workflow count by one, slightly helping to address
#36143

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

.github/actions/runner-timeout/action.yml

@@ -0,0 +1,37 @@
+# In the unlikely event a self-hosted runner was selected and reserved but it
+# goes down before the workload starts, cancel the workflow run.
+name: Detect self-hosted runner assigment timeout
+description: "Cancel the workflow run if a self-hosted runner was selected, but the job failed to start"
+inputs:
+  unique-id:
+    required: true
+    description: "Unique ID of the runner"
+  github_token:
+    required: true
+    description: "Must be able to do github API calls and cancel jobs."
+
+runs:
+  using: "composite"
+  steps:
+    - name: Wait a bit
+      shell: bash
+      run: sleep 120
+
+    - name: Cancel if workload job is still queued
+      shell: bash
+      run: |
+        run_url=/repos/${{ github.repository }}/actions/runs/${{ github.run_id }}
+        export GH_TOKEN=${{ inputs.github_token }}
+  
+        if [ "$(gh api "$run_url/jobs" \
+          | jq -er --arg id '${{ inputs.unique-id }}' \
+            '.jobs[] | select(.name | contains("[" + $id + "]")) | .status'
+        )" = queued ]; then
+          echo 'Timeout waiting for runner assignment!'
+          echo 'Hint: does this repo have permission to access the runner group?'
+          echo 'Hint: https://github.com/organizations/servo/settings/actions/runner-groups'
+          echo
+          echo 'Cancelling workflow run'
+          gh api "$run_url/cancel" --method POST
+          exit 1
+        fi