uncurse mattermost

thanks to emily for the secret nixpkgs knowledge!

pkgs/mattermost.nix

@@ -9,17 +9,13 @@ let
 
     src = "${sources.mattermost-server}/server";
 
-    # this is a hack. with it uncommented, the dependencies as specified by the vendor
-    # hash can be built, but not the package itself. without it, the package itself
-    # can be built, but not its dependencies. Thus the only way to build this package
-    # for now is to run it twice, and rely on the nix store's caching
-    #
-    # this bit is cursed, and i am too tired to try and fix it now
-    #  ~terru
-
-    # preBuild = ''
-    #   go mod tidy
-    # '';
+    # TODO: this should not be needed after the next update, assuming they don't
+    # again push a release without running `go mod tidy`
+    overrideModAttrs = (_: {
+      preBuild = ''
+        go mod tidy
+      '';
+    });
 
     vendorHash = "sha256-ZbLSxG9Gyhk7PBC2V6sMtrQNXvm+ugMfliFIHWO1VLs=";