Allow working out of the box on CI jobs with Github Actions

[alexdiliberto]

Currently, if you run on Travis and leave addon-docs.js empty it will just work because it falls back to process.env.TRAVIS_BRANCH to grab the correct getVersionPath

We should allow the same fallback behavior for Github Actions.

If, when using Github Actions, you don't explicitly set you getVersionPath() or ADDON_DOCS_VERSION_PATH you would get error:

Run yarn ember deploy production
yarn run v1.19.2
$ /home/runner/work/ember-transformicons/ember-transformicons/node_modules/.bin/ember deploy production
no addon-docs version path configured; skipping deploy
undefinedPipeline aborted
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
##[error]Process completed with exit code 1.