buildGoModule: remove unused `with builtins;`

, ... }@args':

with builtins;

assert goPackagePath != "" -> throw "`goPackagePath` is not needed with `buildGoModule`";

assert (vendorSha256 == "_unset" && vendorHash == "_unset") -> throw "either `vendorHash` or `vendorSha256` is required";

assert (vendorSha256 != "_unset" && vendorHash != "_unset") -> throw "both `vendorHash` and `vendorSha256` set. only one can be set.";