gitea: fix undefined variable 'gitea'.

Commit 42c35dea37, which is a cherry-pick
of 28c20a4731da9d5ba539e2d1ef6bcf3ddf1026ac uses the variable 'gitea',
which is not defined in the 18.03 module.

Fix this by: gitea -> pkgs.gitea

nixos/modules/services/misc/gitea.nix

@@ -266,7 +266,7 @@ in
         # update command option in authorized_keys
         if [ -r ${cfg.stateDir}/.ssh/authorized_keys ]
         then
-          sed -ri 's,/nix/store/[a-z0-9.-]+/bin/gitea,${gitea.bin}/bin/gitea,g' ${cfg.stateDir}/.ssh/authorized_keys
+          sed -ri 's,/nix/store/[a-z0-9.-]+/bin/gitea,${pkgs.gitea.bin}/bin/gitea,g' ${cfg.stateDir}/.ssh/authorized_keys
         fi
       '' + optionalString (usePostgresql && cfg.database.createDatabase) ''
         if ! test -e "${cfg.stateDir}/db-created"; then