Support for GitHub Actions

[StanleyGoldman]

Fixes #1931
Fixes #1993

Functionality:

• Detects GitHub Actions as a build server
• Outputs environment variables to future GitHub Actions build steps

Notes:

• Unable to set environment variables for current build step.
• GitHub Actions doesn't have the concept of a build version as I am familiar with it in something like AppVeyor
• Changed a resharper setting which locates imports in a strange position

Using Documentation from:

https://help.github.com/en/actions/automating-your-workflow-with-github-actions/using-environment-variables#default-environment-variables

https://help.github.com/en/actions/automating-your-workflow-with-github-actions/development-tools-for-github-actions#set-an-environment-variable-set-env

[arturcic]

@StanleyGoldman thanks for taking the time to implement, I'll have a look later. One think I did not see is the registration in the DI system. Please add the registration here https://github.com/GitTools/GitVersion/blob/master/src/GitVersionCore/GitVersionCoreModule.cs#L59

[StanleyGoldman]

No problem.

[StanleyGoldman]

I'm still testing this myself, I'll let you know when I feel confident in it.

[StanleyGoldman]

@arturcic I'm ready for your thoughts

[StanleyGoldman]

@arturcic Fixed up things after your review

[StanleyGoldman]

@arturcic fixed up again 😅

[arturcic]

Thank you so much for your contributions, @StanleyGoldman. Thanks for the time you spent 👍

[TingluoHuang]

Should this be GITHUB_ACTIONS?

GITHUB_ACTIONS	Always set to true when GitHub Actions is running the workflow. You can use this variable to differentiate when tests are being run locally or by GitHub Actions.

[arturcic]

Makes sense, are you in a position to send a PR with the fix?

[TingluoHuang]

#2006