Add build cache to CI workflow

NOTE: This requires an update to your build container. See
e132563a5a

This normalizes file modtimes with git-restore-mtime, and caches
the build directory. Subsequent runs will only rebuild the files
that have changed.

.github.example/workflows/build.yml

@@ -22,15 +22,30 @@ jobs:
     - name: Checkout
       uses: actions/checkout@v4
       with:
+        fetch-depth: 0
         submodules: recursive
 
     # Set Git config
     - name: Git config
       run: git config --global --add safe.directory "$GITHUB_WORKSPACE"
 
+    # Normalize file mod times
+    - name: Restore timestamps
+      run: uv run https://raw.githubusercontent.com/MestreLion/git-tools/refs/tags/v2022.12/git-restore-mtime
+
     # Copy the original files to the workspace
     - name: Prepare
-      run: cp -R /orig .
+      run: cp -a /orig .
+
+    # Restore cached files
+    - name: Cache build
+      uses: actions/cache@v4
+      with:
+        path: |
+          build
+          .ninja_deps
+          .ninja_log
+        key: ${{ runner.os }}-${{ matrix.version }}
 
     # Build the project
     - name: Build