Linters workflow for GitHub Actions fails for forks

[javierm]

Current behavior

Currently, when running our Linters workflow in GitHub Actions, we're getting an error in pull requests opened from other repositories:

bundler: failed to load command: pronto (/home/runner/work/consuldemocracy/consuldemocracy/vendor/bundle/ruby/3.1.0/bin/pronto)
/home/runner/work/consuldemocracy/consuldemocracy/vendor/bundle/ruby/3.1.0/gems/pronto-0.11.2/lib/pronto/git/repository.rb:99:in `merge_base': object not found - no match for id  (Rugged::OdbError)

Expected behavior

Ideally, Pronto will add comments to the pull request just like it does when running this workflow in pull requests opened using a branch from this repository.

If that isn't possible, the behavior we had years ago is also acceptable: the job fails when there are errors and suceeds when there are no errors, although no comments are added to the pull request.

Notes

The current error might be related to prontolabs/pronto#404. In the past, we've run into other errors, so solving this one might not be enough to solve the issue.

Maybe we should skip Pronto in these cases and only run Rubocop and ESLint on every file 🤔. Right now we can't run ERBLint and Stylelint because the existing code has issues, though, which is the whole reason why we started using Pronto in the first place. And we wouldn't get information about the "refactor" Rubocop rules either.