ci: run approved fork PR compile jobs on the disposable fork runner (#2246)

* ci: run approved fork PR compile jobs on the disposable fork runner

* ci: ignore fork runner heartbeats from the future

* ci: target the fork runner by its own label

* Read the fork runner heartbeat from a public gist

* Fall back to an anonymous read of the fork runner heartbeat

.github/workflows/pick-runner.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-24.04
     timeout-minutes: 2
     outputs:
-      labels: ${{ steps.runner.outputs.labels || '"ubuntu-24.04"' }}
+      labels: ${{ steps.runner.outputs.labels || steps.fork.outputs.labels || '"ubuntu-24.04"' }}
     steps:
       - name: Pick the compile runner
         id: runner
@@ -36,3 +36,25 @@ jobs:
           fi
           echo "labels=$labels" >> "$GITHUB_OUTPUT"
           echo "Compile runner: \`$labels\`" >> "$GITHUB_STEP_SUMMARY"
+      - name: Pick the fork runner
+        id: fork
+        if: github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository
+        continue-on-error: true
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          set -euo pipefail
+          labels='"ubuntu-24.04"'
+          gist=https://api.github.com/gists/0c7ed7ad6d6493de84f5123689c64ae6
+          body=$(curl -fsS --max-time 15 -H "Authorization: Bearer $GH_TOKEN" "$gist" 2>/dev/null || curl -fsS --max-time 15 "$gist" 2>/dev/null || echo '{}')
+          IDLE_AT=$(jq -r 'select(.owner.login == "maxjivi05") | .files["idle-at"].content // "0"' <<< "$body" 2>/dev/null || echo 0)
+          echo "Heartbeat: ${IDLE_AT:-none}"
+          age=-1
+          if [[ "${IDLE_AT:-0}" =~ ^[0-9]+$ ]] && [ "${IDLE_AT:-0}" -gt 0 ]; then
+            age=$(( $(date +%s) - IDLE_AT ))
+          fi
+          if [ "$age" -ge 0 ] && [ "$age" -lt 180 ]; then
+            labels='"droiddeck-fork"'
+          fi
+          echo "labels=$labels" >> "$GITHUB_OUTPUT"
+          echo "Compile runner: \`$labels\`" >> "$GITHUB_STEP_SUMMARY"