CI: drastically reduce the number of tests for package only PRs (#22410)

PRs that change only package recipes will only run tests under "package_sanity.py" and without coverage. This should result in a huge drop the cpu-time spent in CI for most PRs.

.github/workflows/unit_tests.yaml

@@ -332,7 +332,7 @@ jobs:
           coverage xml
         else
           echo "ONLY PACKAGE RECIPES CHANGED [skipping coverage]"
-          $(which spack) unit-test -x -m "not maybeslow"
+          $(which spack) unit-test -x -m "not maybeslow" -k "package_sanity"
         fi
     - uses: codecov/codecov-action@v1
       if: ${{ needs.changes.outputs.with_coverage == 'true' }}