This update for golang-github-google-jsonnet fixes the following issues:

Update to version v0.20.0:

* Update cpp version (#687)
* Add `std.round` function in standard library. (#683)
* Implement std.isEmpty for string (#678)
* Add std.xor for 2 booleans
* Add support to return int based value from native functions golang implementations
* Implement std.xor for booleans
* feat: implement std.sum