Add pre-publish step to check validity of git auth

[ecraig12345]

The publish flow for beachball is currently as follows:

1. bump packages locally
2. publish to npm
3. push to git

Currently when publishing packages, if the git auth is invalid (due to an expired token, incorrect URL, or other issue), this will result in the repo being out of sync with the published versions.

Therefore, it would be good to add a step prior to npm publish to check the validity of the github auth. I'm not sure what the best command would be to do this, since it appears that git fetch (which is run prior to publishing) succeeds for public repos even with an expired token.