Avoid run tests on branch `gh-pages` in Heroku

[jefer94]

The gh-pages is managed by MkDocs, I not want every time MkDocs is deployed with Github Actions with the command mkdocs gh-deploy --force, because that branch never ever would contains tests, that cause the CI in Heroku fail each time this branch is updated, but that is a bad approach and ends by waste the resources and money of Heroku and me