ci: Do not run automatic homebrew cleanup on install (#674)

Currently, the macOS CI job spends a considerable amount of time cleanup
up the homebrew install, which is pointless because it's erased at the
end of the job anyway. Adding an environment variable, this can be
disabled.

Co-authored-by: robertlangenberg <56069170+robertlangenberg@users.noreply.github.com>

.github/workflows/builds.yml

@@ -9,6 +9,7 @@ on:
 
 env:
   CTEST_OUTPUT_ON_FAILURE: 1
+  HOMEBREW_NO_INSTALL_CLEANUP: 1
 
 jobs:
   linux: