buildBazelPackage: don't fail if the marker file doesn't exist

This was the intent of my previous change, but it didn't quite work
because the pipeline still exits false even if the file doesn't exist.
Oops.

pkgs/build-support/build-bazel-package/default.nix

···
       find $bazelOut/external -maxdepth 1 -type l | while read symlink; do
         name="$(basename "$symlink")"
         rm "$symlink"
-        test -f "$bazelOut/external/@$name.marker" && rm "$bazelOut/external/@$name.marker"
+        test -f "$bazelOut/external/@$name.marker" && rm "$bazelOut/external/@$name.marker" || true
       done
 
       # Patching symlinks to remove build directory reference