Support fork for GitHub Checks

[j0k3r]

Creating a PR from a fork of the repo doesn't trigger GitHub Checks for the bundle analyzer.

For example:

• that one coming from a fork has no checks: https://github.com/smooth-code/bundle-analyzer/pull/20
• that one coming from a branch of the repo has checks: https://github.com/smooth-code/bundle-analyzer/pull/12

Following https://twitter.com/j0k/status/1185189452307718145