ci: re-add gpu variant now that base builds green

base builds end-to-end on ubuntu-22.04 (slirp networking, KVM accel) and
passes the static smoke. Restore the [base, gpu] matrix; fail-fast:false
keeps base green while the gpu driver bake is validated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: paulgessinger

.github/workflows/build-images.yml

@@ -34,7 +34,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        variant: [base]   # TEMP: base only until CI is green; gpu re-added after
+        variant: [base, gpu]
 
     steps:
       - uses: actions/checkout@v5