fix system builds with binary cache disabled

nix/coordinator.nix

@@ -160,6 +160,7 @@ in
 
     # TODO(raito): we assume worker runs on coordinator. please clean up this later.
     systemd.services.buildbot-worker.serviceConfig.Environment =
+      lib.mkIf cfg.binaryCache.enable (
         let
           awsConfigFile = pkgs.writeText "config.ini"  ''
             [default]
@@ -170,7 +171,8 @@ in
         [
           "AWS_CONFIG_FILE=${awsConfigFile}"
           "AWS_SHARED_CREDENTIALS_FILE=${cfg.binaryCache.profileCredentialsFile}"
-        ];
+        ]
+      );
 
     systemd.services.buildbot-master = {
       after = [ "postgresql.service" ];