We use TMPDIR, not TEMP_DIR.

wiki/src/contribute/release_process/test/usage.mdwn

@@ -90,7 +90,7 @@ by the local configuration file:
   after pressing ENTER. This is useful for updating outdated images,
   or when extracting new images. Defaults to `false`.
 
-* `TEMP_DIR`: String value. Directory where various temporary files
+* `TMPDIR`: String value. Directory where various temporary files
   are written during a test, e.g. VM snapshots and memory dumps,
   failure screenshots, pcap files and disk images. Defaults to
   `"/tmp/TailsToaster"`.