Fix mutation testing on dev branch

Author: staabm

.github/workflows/build.yml

@@ -207,8 +207,14 @@ jobs:
           > infection.json5
           cat infection.json5 | jq
 
-      - name: "Cache Result cache"
-        uses: actions/cache@v4
+      - name: "Determine default branch"
+        id: default-branch
+        working-directory: phpstan-dist
+        run: |
+          echo "name=$(git remote show origin | sed -n '/HEAD branch/s/.*: //p')" >> $GITHUB_OUTPUT
+
+      - name: "Restore result cache"
+        uses: actions/cache/restore@v4
         with:
           path: ./tmp
           key: "result-cache-v1-${{ matrix.php-version }}-${{ github.run_id }}"
@@ -217,13 +223,20 @@ jobs:
 
       - name: "Run infection"
         run: |
-          git fetch --depth=1 origin $GITHUB_BASE_REF
+          git fetch --depth=1 origin ${{ steps.previous-commit.outputs.name }}
           infection \
-            --git-diff-base=origin/$GITHUB_BASE_REF \
+            --git-diff-base=origin/${{ steps.previous-commit.outputs.name }} \
             --git-diff-lines \
             --ignore-msi-with-no-mutations \
             --min-msi=100 \
             --min-covered-msi=100 \
             --log-verbosity=all \
             --debug \
             --logger-text=php://stdout
+
+      - name: "Save result cache"
+        uses: actions/cache/save@v4
+        if: ${{ !cancelled() }}
+        with:
+          path: ./tmp
+          key: "result-cache-v1-${{ matrix.php-version }}-${{ github.run_id }}"