Quote shell variable values (#12759)

This updates logic which sets shell variables to quote the values,
which is necessary when the value contains a space (e.g. PATH).

lib/spack/spack/util/environment.py

@@ -492,8 +492,8 @@ def shell_modifications(self, shell='sh'):
                 if new is None:
                     cmds += _shell_unset_strings[shell].format(name)
                 else:
-                    cmds += _shell_set_strings[shell].format(name,
-                                                             new_env[name])
+                    cmds += _shell_set_strings[shell].format(
+                        name, cmd_quote(new_env[name]))
         return cmds
 
     @staticmethod