upgrade: allow enable/disable manifest TAM for unencrypted repos

Recent borg wrote TAM authenticated **archives**
even for unencrypted repos (encryption "none"),
so we also do that for the manifest.

It's kind of fake as there is no secret key involved then,
but it simplifies the code.

src/borg/archiver.py

@@ -1647,11 +1647,6 @@ class Archiver:
                 cache.commit()
         elif args.tam:
             manifest, key = Manifest.load(repository, (Manifest.Operation.CHECK,), force_tam_not_required=args.force)
-
-            if not hasattr(key, 'change_passphrase'):
-                print('This repository is not encrypted, cannot enable TAM.')
-                return EXIT_ERROR
-
             if not manifest.tam_verified or not manifest.config.get(b'tam_required', False):
                 print('Manifest contents:')
                 for archive_info in manifest.archives.list(sort_by=['ts']):
@@ -1659,7 +1654,7 @@ class Archiver:
                 manifest.config[b'tam_required'] = True
                 manifest.write()
                 repository.commit(compact=False)
-            if not key.tam_required:
+            if not key.tam_required and hasattr(key, 'change_passphrase'):
                 key.tam_required = True
                 key.change_passphrase(key._passphrase)
                 print('Key updated')
@@ -1673,7 +1668,7 @@ class Archiver:
             manifest, key = Manifest.load(repository, Manifest.NO_OPERATION_CHECK, force_tam_not_required=True)
             if tam_required(repository):
                 os.unlink(tam_required_file(repository))
-            if key.tam_required:
+            if key.tam_required and hasattr(key, 'change_passphrase'):
                 key.tam_required = False
                 key.change_passphrase(key._passphrase)
                 print('Key updated')