Refresh virtual environment on Action specification changing

Because the GitHub Action YAML file is currently the only place where
this repository pins the CASE version being tested, have any changes to
that file trigger a virtual environment rebuild, which locally (to a
developer's desktop) re-runs the test.

Signed-off-by: Alex Nelson <[email protected]>

Makefile

@@ -58,6 +58,7 @@ all: \
 	touch $@
 
 .venv.done.log: \
+  .github/workflows/ci.yml \
   pyproject.toml
 	rm -rf venv
 	$(PYTHON3) -m venv \