CI: Enable native multi-image collectives testing

Author: bonachea

.github/workflows/build.yml

@@ -41,7 +41,7 @@ jobs:
             version: latest
             network: smp
             native_multi_image: 1
-            FFLAGS: -fcoarray -DHAVE_SYNC=0 -DHAVE_COLLECTIVES=0
+            FFLAGS: -fcoarray -DHAVE_SYNC=0
             # https://hub.docker.com/r/snowstep/llvm/tags
             container: snowstep/llvm:bookworm
           - os: ubuntu-24.04