snowcrash: remove unnecessary `runVend`

also disable `postFixup` for darwin which is case insensitive

pkgs/tools/security/snowcrash/default.nix

···
 { lib
+, stdenv
 , buildGoModule
 , fetchFromGitHub
 }:
···
     sha256 = "16p1nfi9zdlcffjyrk1phrippjqrdzf3cpc51dgdy3bfr7pds2ld";
   };
 
-  vendorSha256 = "1xm2yjr4mqkara3yib6vgfj14ldh7r0v1vr2i0ks13l1rm54x840";
+  vendorSha256 = "sha256-YryQKLHwUDhR/Z7eWfCdL86Z83GrqBTvdGGj+dGKvjI";
 
   subPackages = [ "." ];
 
-  runVend = true;
-
-  postFixup = ''
+  postFixup = lib.optionals (!stdenv.isDarwin) ''
     mv $out/bin/SNOWCRASH $out/bin/${pname}
   '';