Remove default branch determination from workflow

Author: ondrejmirtes

.github/workflows/build.yml

@@ -235,15 +235,6 @@ jobs:
           > infection.json5
           jq . infection.json5
 
-      - name: "Determine default branch"
-        id: default-branch
-        run: |
-          BRANCH=$(git remote show origin | sed -n '/HEAD branch/s/.*: //p')
-          delimiter="$(openssl rand -hex 16)"
-          echo "name<<${delimiter}" >> "$GITHUB_OUTPUT"
-          echo "$BRANCH" >> "$GITHUB_OUTPUT"
-          echo "${delimiter}" >> "$GITHUB_OUTPUT"
-
       - name: "Restore result cache"
         uses: actions/cache/restore@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0
         with:
@@ -254,9 +245,9 @@ jobs:
 
       - name: "Run infection"
         run: |
-          git fetch --depth=1 origin ${{ steps.default-branch.outputs.name }}
+          git fetch --depth=1 origin "$GITHUB_BASE_REF"
           infection \
-            --git-diff-base=origin/${{ steps.default-branch.outputs.name }} \
+            --git-diff-base=origin/"$GITHUB_BASE_REF" \
             --git-diff-lines \
             --ignore-msi-with-no-mutations \
             --min-msi=100 \