Use azpysdk ApiStub in CI

[JennyPng]

#42601

• When using uv venv, it errored with "module pip not found", so I believe apistubgen uses pip
• To address ^, i added an ensurepip call during venv creation if using uv
• was also seeing some import error with two core packages so I switched to run the apistub command from the staging directory instead of package_dir, which resolved it 🐱

pipelines:

• core: https://dev.azure.com/azure-sdk/internal/_build/results?buildId=5688881&view=logs&j=4259cd7e-6a4b-5ca5-581a-0ae020cd9d20&t=425fe497-0bba-57e5-9867-ebf1b63abedf
• core nightly: https://dev.azure.com/azure-sdk/internal/_build/results?buildId=5688998&view=logs&j=4259cd7e-6a4b-5ca5-581a-0ae020cd9d20&t=425fe497-0bba-57e5-9867-ebf1b63abedf
• storage:
• storage nightly: