Fix depcheck workflow to not fail from dependabot

[patcon]

The logic that was supposed to skip steps when a GH token was unavailable was working for users submitting PRs from forks, but not for PRs from dependabot. This results in noisy failures.

• This PR will confirm that steps are properly skipped when GitHub secret token isn't available.
• https://github.com/patcon/polis/pull/402 confirmed that steps are run successfully when token is available.
• Confirming it works for dependabot PRs will require merging and waiting for it to submit its next one.

[patcon]

K, got this working properly. Issues should be gone