github/workflows: Reduce amount of times we do build_tests.yaml
github/workflows: Reduce amount of times we do build_tests.yaml

Rather than doing this action whenever _anything_ in .github/workflows
changes, limit it to just changes to this action.

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@digital-domain.net>

ac000/libac