feat(config): Try SCOOP_GH_TOKEN value first before gh_token value from config

[ghost]

Description

There are multiple usages of GitHub token value in the scoop (in checkver.ps1, core.ps1, install.ps1), but only in once place (in checkver.ps1), there is a logic to try using token value from SCOOP_GH_TOKEN environment variable first. This PR unifies the token value obtain logic and puts it into the function, which is used everywhere token value is needed.

Motivation

Using token in environment variable is needed for GitHub Actions setup (I am setting up GitHub Actions for private bucket right now and token is required for accessing artifacts located in private repository).

How Has This Been Tested?

Tested with private GitHub repositories using both gh_token and SCOOP_GH_TOKEN values.

Checklist:

• I have read the Contributing Guide.
• I have updated the documentation accordingly.
• I have updated the tests accordingly.

[rasa]

Untested but LGTM