[CI] Figure out a way to run GitHub Actions from forks

[nolanlawson]

Right now, if a PR is opened from a fork, it will run the GitHub Actions and fail because of missing SauceLabs credentials (due to secrets). This also occurs when Dependabot opens PRs (even though it's not a fork – the Dependabot account does not have sufficient credentials apparently).

My current workaround is to either push an empty commit to the branch or open a new PR with the exact same changes, but this is time-consuming and tedious. There has got to be a better way to do this.