draft PR because for some reason the token provided by GitHub isn't recognized as valid

see for example https://github.com/boegel/easybuild-framework/actions/runs/4181752996/jobs/7244026081

$ eb --check-github
...
* GitHub user...easybuild_test => OK
* GitHub token...ghp..FGM (len: 40) => FAIL (validation failed)

@migueldiascosta pointed out to me that we're using ${{secrets.TEST_GITHUB_TOKEN}}, which is a self-defined environment variable, so that probably has a now expired GitHub token...

We should try using the standard GITHUB_TOKEN instead (see https://docs.github.com/en/actions/security-guides/automatic-token-authentication)

Broken tests should all be fixed now, and all GitHub tests pass again when a GitHub token is available (most GitHub tests are skipped when no GitHub token is available, when the test suite is run in the scope of a PR for example, like here). See https://github.com/boegel/easybuild-framework/actions/runs/4698932377 for a run where a GitHub token is available and no tests are skipped.

I had to go back to provided a self-created GitHub token and make it available to the tests as an encrypted secret, because some tests require that the token has gist permissions, for example.

cc @migueldiascosta