Update tests.yml

Author: staabm

.github/workflows/tests.yml

@@ -267,6 +267,11 @@ jobs:
           > infection.json5
           cat infection.json5 | jq
 
+      - name: "Determine default branch"
+        id: default-branch
+        run: |
+          echo "name=$(git remote show origin | sed -n '/HEAD branch/s/.*: //p')" >> $GITHUB_OUTPUT
+
       - name: "Restore result cache"
         uses: actions/cache/restore@v4
         with:
@@ -277,9 +282,9 @@ jobs:
 
       - name: "Run infection"
         run: |
-          git fetch --depth=1 origin $GITHUB_BASE_REF
+          git fetch --depth=1 origin ${{ steps.default-branch.outputs.name }}
           infection \
-            --git-diff-base=origin/$GITHUB_BASE_REF \
+            --git-diff-base=origin/${{ steps.default-branch.outputs.name }} \
             --git-diff-lines \
             --ignore-msi-with-no-mutations \
             --min-msi=100 \