Bug 1620936 - Avoid creating a virtualenv when running unpack for diffoscope. r=froydnj

Creating the virtualenv prints an error message in the diffoscope docker
image because it doesn't have a compiler or the python development
headers, and that triggers an error setting up psutil after bug 1620513,
which in turn leads to a noisy and unrelated error message appearing in
the diffoscope jobs when they fail.

We don't need psutil set up really in those jobs, let alone a full
fledged virtualenv, so we can avoid the psutil error message by
disabling the virtualenv.

Differential Revision: https://phabricator.services.mozilla.com/D66361

--HG--
extra : moz-landing-system : lando

taskcluster/docker/diffoscope/get_and_diffoscope

@@ -98,7 +98,7 @@ for option; do
 		for dir in a b; do
 			# Need to run mach python from inside the gecko source.
 			# See bug #1533642.
-			(cd $GECKO_PATH && ./mach python toolkit/mozapps/installer/unpack.py --omnijar $OMNIJAR $CURDIR/$dir)
+			(cd $GECKO_PATH && ./mach python --no-virtualenv toolkit/mozapps/installer/unpack.py --omnijar $OMNIJAR $CURDIR/$dir)
 		done
 		;;
 	--fail)