ci: fix gitignore to allow prebuilt cache files, use git add -f

Author: pepc84

.github/workflows/build-cache.yml

@@ -173,6 +173,6 @@ jobs:
         run: |
           git config user.name "github-actions[bot]"
           git config user.email "github-actions[bot]@users.noreply.github.com"
-          git add cache/
+          git add -f cache/
           git diff --staged --quiet || git commit -m "ci: update precompiled cache (Processing.o + PCH) for all platforms"
           git push

.gitignore

@@ -1,2 +1,5 @@
 .zip
-cache/
+# Allow prebuilt platform cache files (committed by CI)
+cache/**/*.tmp
+cache/**/_macro_preprocess_input.cpp
+