ci: check out the tools submodule

reweave/build's `make all` also builds tools/runner from the tools
submodule (weaveworks/build-tools); run 7 compiled every weave binary and
then stopped there. Fetched from upstream for now, like the build's other
fetches.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UHXzGrsxfkGk8LmwrBbHNZ

.gitea/workflows/images.yml

@@ -30,6 +30,7 @@ jobs:
       - uses: actions/checkout@v7
         with:
           fetch-depth: 0 # tags, for the version string
+          submodules: true # tools/ (weaveworks/build-tools): make all builds tools/runner
 
       - name: Log in to the registry
         run: echo "${{ secrets.REGISTRY_TOKEN }}" | docker login "$REGISTRY" -u reweave-ci --password-stdin