ci: use virtualenv to avoid race conditions

[jakubgs]

Desktop QA tests also use Pytest packages and their versions are different, so we can't install them globally, it needs to be done per build using WORKSPACE_TMP as destination.

Otherwise you get this due to a race condition:

 > a ci-slave-linux,ci-slave-release -o -a 'sudo -u jenkins pip list --user 2>/dev/null | grep "^pytest "' | sort
linux-01.he-eu-hel1.ci.release | CHANGED | rc=0 | (stdout) pytest  7.4.0
linux-02.he-eu-hel1.ci.release | CHANGED | rc=0 | (stdout) pytest  7.4.0
linux-01.he-eu-hel1.ci.devel | CHANGED | rc=0 | (stdout) pytest    7.2.1
linux-02.he-eu-hel1.ci.devel | CHANGED | rc=0 | (stdout) pytest    7.4.0
linux-03.he-eu-hel1.ci.devel | CHANGED | rc=0 | (stdout) pytest    7.4.0
linux-04.he-eu-hel1.ci.devel | CHANGED | rc=0 | (stdout) pytest    7.4.0
linux-05.he-eu-hel1.ci.devel | CHANGED | rc=0 | (stdout) pytest    7.2.1

Related to:

• ci: use virtualenv to avoid race conditions status-mobile#19637