nixos/murmur: Set UMask to 027

Group only needs limited access, while other users don't need access at
all. So set the UMask to 027.

Signed-off-by: Felix Singer <felixsinger@posteo.net>

nixos/modules/services/networking/murmur.nix

@@ -349,6 +349,7 @@ in
         RestrictRealtime = true;
         SystemCallArchitectures = "native";
         SystemCallFilter = "@system-service";
+        UMask = 027;
       };
     };