Update `codegen-library.sh` to set exec flag on new k8s.io/code-generator scripts

[andrew-su]

As of v0.28.0 of k8s.io/code-generator the generate-groups.sh script calls generate-internal-groups.sh. We should update our script to additionall add the exec flag for the generate-internal-groups.sh file.

https://github.com/kubernetes/code-generator/blob/791c213a776b4b92f9af0805c1cf57c118ba951c/generate-groups.sh#L66

[andrew-su]

Related: #336