fix: revert to PID-based temp files (was working before)

Reverts the unnecessary mktemp change from 638cb0afe that broke cache saving.

What happened:
- Original delta code used $$ (PID) for temp files: DELTA_TARBALL="/tmp/...-$$.tar.zst"
- This creates a STRING, not a file - zstd creates the file when writing
- When removing deltas (638cb0afe), I unnecessarily changed to mktemp for "better practice"
- mktemp CREATES an empty file - zstd refuses to overwrite it
- Result: "already exists; not overwritten" error

Why it seemed to work:
- Immutability check skipped save for existing caches
- Upload code path never executed during testing
- Bug only appeared when actually trying to create new cache

The fix:
- Revert to PID-based naming ($$) that was working
- Don't fix what isn't broken

Applies to both save and restore actions for consistency.

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

@@ -170,7 +170,7 @@ runs:
 
         # Download and extract cache
         S3_KEY="s3://${S3_BUCKET}/${MATCHED_KEY}-base.tar.zst"
-        TEMP_TARBALL=$(mktemp /tmp/xahau-cache-XXXXXX.tar.zst)
+        TEMP_TARBALL="/tmp/xahau-cache-restore-$$.tar.zst"
 
         echo ""
         echo "Downloading cache..."