scripts: check in library generation script

[kevinburkesegment]

Also update build_twilio_library.py to use subprocess instead of shelling out to os.system.

Fixes

A short description of what this PR does.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• Run make test-docker
• Verify affected language:
  • Generate twilio-go from our OpenAPI specification using the build_twilio_go.py using python examples/build_twilio_go.py path/to/twilio-oai/spec/yaml path/to/twilio-go and inspect the diff
  • Run make test in twilio-go
  • Create a pull request in twilio-go
  • Provide a link below to the pull request
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, please create a GitHub Issue in this repository.