Installations: don't set group permissions when they match what is desired (#38036)

* When installing a package Spack will attempt to set group permissions on
the install prefix even when the configuration does not specify a group.

Co-authored-by: David Gomez <dvdgomez@users.noreply.github.com>

lib/spack/llnl/util/filesystem.py

@@ -610,6 +610,8 @@ def chgrp(path, group, follow_symlinks=True):
         gid = grp.getgrnam(group).gr_gid
     else:
         gid = group
+    if os.stat(path).st_gid == gid:
+        return
     if follow_symlinks:
         os.chown(path, -1, gid)
     else: