Add ubuntu-rootfs full-golden toolcache selftest workflow

Dispatchable smoke test that verifies the baked-in runner-images tool cache in
the --toolset full golden is present and consumed by setup-go/node/python
(versions resolve locally instead of downloading).

Author: linyiru

.github/workflows/ubuntu-rootfs-selftest.yml

@@ -0,0 +1,36 @@
+name: ubuntu-rootfs-selftest
+
+# Smoke test for the "full" ubuntu-rootfs golden (build-ubuntu-rootfs.sh
+# --toolset full): confirms the baked-in actions/runner-images tool cache is
+# present and consumed by the setup-* actions (versions resolve locally instead
+# of downloading). Dispatch against a firerunner listener booting the full
+# golden. Versions below match toolset-2404.json; adjust if the ref changes.
+on:
+  workflow_dispatch:
+
+jobs:
+  toolcache:
+    runs-on: firerunner
+    steps:
+      - uses: actions/checkout@v4
+      - name: setup-go (from go.mod)
+        uses: actions/setup-go@b7ad1dad31e06c5925ef5d2fc7ad053ef454303e # v7.0.0
+        with:
+          go-version-file: go.mod
+          cache: false
+      - name: setup-node
+        uses: actions/setup-node@v4
+        with:
+          node-version: '24'
+      - name: setup-python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.13'
+      - name: report
+        run: |
+          echo "RUNNER_TOOL_CACHE=${RUNNER_TOOL_CACHE:-unset}"
+          echo "AGENT_TOOLSDIRECTORY=${AGENT_TOOLSDIRECTORY:-unset}"
+          echo "go:     $(go version)"
+          echo "node:   $(node --version)"
+          echo "python: $(python --version)"
+          echo "cache tools: $(ls "${RUNNER_TOOL_CACHE:-/opt/hostedtoolcache}" 2>/dev/null | tr '\n' ' ')"