builder: use correct secret when fetching token

Use the `self.secret` and not `self.id` for the secret. Doh. Mea culpa.
Fix the corresponding test as well, which also checked for the wrong
thing.

Reported-By: Ondřej Budai <ondrej@budai.cz>

plugins/builder/osbuild.py

@@ -298,7 +298,7 @@ class OAuth2(requests.auth.AuthBase):
         data = {
             "grant_type": "client_credentials",
             "client_id": self.id,
-            "client_secret": self.id
+            "client_secret": self.secret
         }
 
         res = http.post(self.token_url, data=data)