gbuild: More crude hacking to fix over-evaluation of gb_ICU_PRECOMMAND

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

Change-Id: I43ff930267cfd3cc537e79c41a8b4167fd5c77d3