feat: enable metacopy=on to test metadata-only copy-up

Mount OverlayFS with metacopy=on option (kernel 4.2+, supported on ubuntu-22.04).
This prevents full file copy-up when files are opened with O_RDWR but not modified.

Expected behavior:
- ccache opens cache files with write access
- OverlayFS creates metadata-only entry in upper layer
- Full copy-up only happens if data is actually written
- Should dramatically reduce delta sizes from ~324 MB to ~KB

Re-enabled use-deltas for ccache to test this optimization.
Conan remains base-only (hash-based keys mean exact match most of the time).

If successful, deltas should be tiny for cache hit scenarios.

.github/actions/xahau-actions-cache-restore/action.yml

@@ -274,11 +274,12 @@ runs:
           exit 0
         fi
 
-        # Mount OverlayFS
+        # Mount OverlayFS with metacopy=on to prevent copy-up for files opened-but-not-written
+        # This dramatically reduces delta sizes when ccache opens files with O_RDWR but doesn't modify content
         echo ""
-        echo "Mounting OverlayFS..."
+        echo "Mounting OverlayFS with metacopy=on..."
         sudo mount -t overlay overlay \
-          -o lowerdir="${CACHE_WORKSPACE}/base",upperdir="${CACHE_WORKSPACE}/upper",workdir="${CACHE_WORKSPACE}/work" \
+          -o lowerdir="${CACHE_WORKSPACE}/base",upperdir="${CACHE_WORKSPACE}/upper",workdir="${CACHE_WORKSPACE}/work",metacopy=on \
           "${CACHE_WORKSPACE}/merged"
 
         # Verify mount