std.jsonnet dependecy not found / cpp-jsonnet sub repo pull unsuccessful

Updating the jsonnet dependencies with ./update_cpp_jsonnet.sh does not work.

The ast generation command fails due to the missing std.jsonnet file.

```bash
go run cmd/dumpstdlibast/dumpstdlibast.go cpp-jsonnet/stdlib/std.jsonnet > astgen/stdast.go
```
Building the jsonnet binary works nevertheless, but the stdlib is outdated.

Replacing 'git pull' with 'git clone https://github.com/google/jsonnet' in ./update_cpp_jsonnet.sh does the job.

I'm not sure why the pull operation is not working, if it's a problem on my side or with the subrepo.

However, fixing this issue should also fix https://github.com/google/go-jsonnet/issues/571. After the described change above the option was available for me.