Use test registry in npmrc for alpha packages

[hallipr]

Currently, the Autorest Regen Preview pipeline for autorest.csharp publishes alpha versions of the c# emitter package to an alternate package feed. This is breaking the validation builds of azure-sdk-for-net because the alpha version specified doesn't exist in the default public repository.

To fix this, if we detect an alpha package version number in emitter-package.json, we create an npmrc file prior to running the code generation check.

[hallipr]

To verify, I ran in azure-sdk-for-net using these pipelines:

Should pass because doesn't use alpha package:
https://dev.azure.com/azure-sdk/public/_build/results?buildId=2799062&view=results

Should fail because uses alpha package and didn't regenerate code in branch:
https://dev.azure.com/azure-sdk/public/_build/results?buildId=2799252&view=results

Should pass because uses alpha package and did regenerate code in branch:
https://dev.azure.com/azure-sdk/public/_build/results?buildId=2801691&view=results

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow