CI: Change flang-latest to use snowstep:bookworm container

Author: bonachea

.github/workflows/build.yml

@@ -43,7 +43,7 @@ jobs:
             native_multi_image: 1
             FFLAGS: -fcoarray -DHAVE_SYNC=0 -DHAVE_COLLECTIVES=0
             # https://hub.docker.com/r/snowstep/llvm/tags
-            container: snowstep/llvm:focal
+            container: snowstep/llvm:bookworm
           - os: ubuntu-24.04
             compiler: flang
             version: 21