bugfix: skip TAM check with BORG_WORKAROUNDS=authenticated_no_key

This is an emergency workaround for authenticated repos
if the user has lost the borg key.

We can't compute the TAM key without the borg key, so just
skip all the TAM stuff.

src/borg/crypto/key.py

@@ -235,6 +235,8 @@ class KeyBase:
         unpacker = get_limited_unpacker('manifest')
         unpacker.feed(data)
         unpacked = unpacker.unpack()
+        if AUTHENTICATED_NO_KEY:
+            return unpacked, True  # True is a lie.
         if b'tam' not in unpacked:
             if tam_required:
                 raise TAMRequiredError(self.repository._location.canonical_path())
@@ -258,8 +260,6 @@ class KeyBase:
         offset = data.index(tam_hmac)
         data[offset:offset + 64] = bytes(64)
         tam_key = self._tam_key(tam_salt, context=b'manifest')
-        if AUTHENTICATED_NO_KEY:
-            return unpacked, True  # True is a lie.
         calculated_hmac = hmac.digest(tam_key, data, 'sha512')
         if not hmac.compare_digest(calculated_hmac, tam_hmac):
             raise TAMInvalid()