Clean up extra directories when we're done with them

If a runner is re-used, the presence of `TMP_DIR` will cause the script
to fail. To be safe, we'll also remove `SVN_DIR` to prevent conflicts,
even though the directories are timestamped.

Fixes #6
1 job for fix/6-tmp-dir-cleanup in 1 minute and 12 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #3449
lint:shell-scripts

00:01:12