minor fixes to caching

.github/workflows/tests.yml

@@ -2,14 +2,14 @@ name: Pull Request Tests
 
 on: [pull_request]
 
-env:
-  # ${{ github.base_ref }}: base branch of the pull request (e.g. master/main).
-  # ${{ github.head_ref }}: branch the pull request targets (e.g. feature branch).
-  TARGET_REF: ${{ github.base_ref }}
 
 jobs:
   test:
     if: "!contains(github.event.head_commit.message, '[skip ci]')"
+    env:
+      BASE_SHA: ${{ github.event_name == 'pull_request' && github.event.pull_request.base.sha || '' }}
+      HEAD_SHA: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
+
     runs-on: ubuntu-latest
 
     steps:
@@ -55,18 +55,20 @@ jobs:
         uses: actions/cache@v4
         with:
           path: ref/coverage_output.txt
-            key: ref-${{ env.TARGET_REF }}
+            key: ref-${{ env.BASE_SHA }}
 
       - name: Checkout ref branch
         if: steps.cache-ref-coverage.outputs.cache-hit != 'true'
         uses: actions/checkout@v4
         with:
           path: ref
-          ref: ${{ env.TARGET_REF }}
+          ref: ${{ github.base_ref }}
 
       - name: Run tests with coverage for ref branch
         if: steps.cache-ref-coverage.outputs.cache-hit != 'true'
-        run: cd ref; pytest --cov=dagrunner --cov-report=term | tee coverage_output.txt
+        run: |
+          cd ref
+          pytest --cov=dagrunner --cov-report=term | tee coverage_output.txt
 
       # TESTS (compare coverage)
       - name: Compare coverage