gbuild: More crude hacking to fix over-evalutation of gb_Extension_*COMMANDs

...in the spirit of 3e70e26cbc96667e2968cd325737053bf8bffb78 "gbuild: fix over-
evaluation in gb_ExternalExecutable__set_internal"

Change-Id: Ica0f8a029e18f66c470afb6a038a4746c919a8d6