GCC package: don't set CC etc. based on bin dir if it doesn't exist (#13000)

7bcb306 updated setup_environment in the GCC package to examine the
bin/ directory to find alternatively-named instances of CC, but this
failed to account for when GCC was being installed and failed when
it attempted to list the not-yet-existing installation prefix. This
commit updates the logic to skip this search if the bin/ directory
doesn't exist.

var/spack/repos/builtin/packages/gcc/package.py

@@ -426,6 +426,10 @@ def setup_environment(self, spack_env, run_env):
 
         # Get the contents of the installed binary directory
         bin_path = self.spec.prefix.bin
+
+        if not os.path.isdir(bin_path):
+            return
+
         bin_contents = os.listdir(bin_path)
 
         # Find the first non-symlink compiler binary present for each language