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-save/action.yml

@@ -76,7 +76,7 @@ runs:
         fi
 
         # Create tarball
-        BASE_TARBALL=$(mktemp /tmp/xahau-cache-XXXXXX.tar.zst)
+        BASE_TARBALL="/tmp/xahau-cache-base-$$.tar.zst"
 
         echo "Creating cache tarball..."
         tar -cf - -C "${TARGET_PATH}" . | zstd -3 -T0 -q -o "${BASE_TARBALL}"