ci: trigger test on root changes

Author: klarkc

.github/workflows/test.yml

@@ -26,7 +26,7 @@ jobs:
         with:
           filters: |
             template:
-              - ${{ matrix.template }}/**
+              - ${{ matrix.template && format('{0}/**', matrix.template) || 'flake.*' }}
       - if: steps.changes.outputs.template == 'true' 
         uses: cachix/install-nix-action@v18
         with: