Bugfix: put environment lock in the right place (#14692)

Locate the environment lock in the hidden environment directory
rather than the root of the environment.

lib/spack/spack/environment.py

@@ -724,7 +724,7 @@ def _transaction_lock_path(self):
         """The location of the lock file used to synchronize multiple
         processes updating the same environment.
         """
-        return os.path.join(self.path, 'transaction_lock')
+        return os.path.join(self.env_subdir_path, 'transaction_lock')
 
     @property
     def lock_path(self):