taskcluster: fix for variable assigment in Makefile
taskcluster: fix for variable assigment in Makefile

mozilla-releng/services,La0/mozilla-relengapi,andrei987/services,La0/mozilla-relengapi,srfraser/services,mozilla-releng/services,La0/mozilla-relengapi,garbas/mozilla-releng-services,andrei987/services,andrei987/services,garbas/mozilla-releng-services,lundjordan/services,garbas/mozilla-releng-services,garbas/mozilla-releng-services,mozilla-releng/services,lundjordan/services,lundjordan/services,srfraser/services,srfraser/services,andrei987/services,srfraser/services,La0/mozilla-relengapi,mozilla-releng/services,lundjordan/services