ci: trust the checkout in Leap container shell steps

Add only GITHUB_WORKSPACE to safe.directory in the Git configuration
used by shell steps. The checkout action configures its own temporary
HOME, leaving subsequent Git commands subject to ownership checks.

Use git status and git diff --exit-code for the generated-files check so
repository errors are not misreported as stale generated files.

Author: mcepl

.github/workflows/build.yml

@@ -59,6 +59,10 @@ jobs:
 
     - uses: actions/checkout@v4
 
+    - name: Trust the checked-out workspace
+      # checkout configures a temporary HOME, not the HOME used by shell steps.
+      run: git config --global --add safe.directory "$GITHUB_WORKSPACE"
+
     - name: Install All Dependencies
       run: sh -x ./.github/posix-deps-zypp.sh
 
@@ -93,12 +97,8 @@ jobs:
     - name: Check for changes in generated files
       if: matrix.task == 'generated-files'
       run: |
-        if ! git diff --quiet; then
-          echo "Generated files are not up to date. Please run 'make regen-all' and commit the changes."
-          git status
-          git diff
-          exit 1
-        fi
+        git status --short
+        git diff --exit-code
 
     - name: Check exported libpython symbols
       if: matrix.task == 'generated-files'