gitlab-ci: Use group instead of subshell

Otherwise the ret variable isn't captured in the parent environment.

.gitlab-ci.yml

@@ -80,7 +80,7 @@ build:
         -A testedPackages
         --keep-going
         --arg bindistTarball $GHC_TARBALL
-        || (echo "Build failed!"; ret=1)
+        || { echo "Build failed!"; ret=1; }
     - scripts/summarize.py || echo "summarize script failed"
     - exit $ret