Self-hosted qa runner has no node in PATH, so the Node-based
actions/checkout@v4 action can't run there. Swap it for a bash git
fetch/checkout, matching the pipeline's no-marketplace-action convention.
Gitea runner build failed with "dotnet: command not found". Added
ensure_dotnet() to lib.sh, using the vendored dotnet-install.sh to
bootstrap the SDK into .dotnet/ when not already on PATH, called from
build.sh/test.sh/prepush.sh. Also trigger CI on build-runner-fix to
verify the fix.