nixos/gitlab: fix registry.issuer setting

Prior to this change, the configuration value for
`services.gitlab.registry.issuer` was only referenced by the
docker-registry configuration and in the `gitlab-registry-cert` service
while the gitlab config used the hard-coded value "gitlab-issuer".

nixos/modules/services/misc/gitlab.nix

@@ -168,7 +168,7 @@ let
         port = cfg.registry.externalPort;
         key = cfg.registry.keyFile;
         api_url = "http://${config.services.dockerRegistry.listenAddress}:${toString config.services.dockerRegistry.port}/";
-        issuer = "gitlab-issuer";
+        issuer = cfg.registry.issuer;
       };
       extra = {};
       uploads.storage_path = cfg.statePath;