Clean up `TMP_DIR`

The script doesn't delete the TMP_DIR when it completes, either successfully or when it aborts due to a gitref mismatch. If a runner is re-used for a subsequent build, it will fail due to TMP_DIR already existing.

• Randomize TMP_DIR name and create as a subdirectory of current TMP_DIR (mkdir -p)
• Introduce cleanup function to remove TMP_DIR
• Call cleanup function before early exits and script completion