[CLI] Fix Github Token handling

[jhnstn]

This uses the same function go-gh uses to find the users Github auth token when cloning repos. Previously it was only looking for GITHUB_TOKEN. This change will allow users to just run gh auth login to use the tool or it will work by supplying a github token via GH_TOKEN or GITHUB_TOKEN if gh is not installed

Testing

With out gh

1. Uninstall gh ( brew uninstall gh)
2. Make sure gh has not added anything to ~/.gitconfig look for something like

   [credential "https://github.com"]
   helper = !/opt/homebrew/bin/gh auth git-credential

3. Make sure ~/.config/gh/hosts.yml does not have an oauth_token
4. Try creating a test release go run main.go release prepare gb <version> --no-tag
5. Note that the command fails
6. Try using a github PAT by adding that as an env :

   GH_TOKEN=<your github token> go run main.go release prepare gb <version> --no-tag
   // or
   GITHUB_TOKEN=<your gitub token> go run main.go release prepare gb <version> --no-tag

7. Verify that the changes can be pushed to github and that the PR is created

With gh installed

1. re-install gh
2. Try without logging in by repeating steps 5 and 6 above
3. Login in via gh auth login and make sure to not run gh auth setup-git
4. Verify that gh has not modified ~./gitconfig
5. Try creating a test release without the exported token: go run main.go release prepare gb <version> --no-tag
6. Verify that the changes can be pushed and that the PR is created