Use GITHUB_TOKEN instead of PAT in "Slash Command Dispatch" and "GMT Dev Tests" workflows

[seisman]

Description of proposed changes

Based on the comments in peter-evans/slash-command-dispatch#147, it's possible to use GITHUB_TOKEN in slash command workflows. This PR removes the step for generating a PAT and uses GITHUB_TOKEN instead, which can simplify our workflows.

Since the slash commands can only run the workflows in the main branch, we can't test the changes without merging the PR.

I've committed the same changes to the main branch in my fork (https://github.com/seisman/pygmt) and tried the slash commands in PR https://github.com/seisman/pygmt/pull/1. Both /test-gmt-dev and /format work (workflow run https://github.com/seisman/pygmt/actions/runs/7393878677 and commit 574e029).

We still need to make sure that the slash commands work for new contributors without written permission and also work for PRs from forks. So, need your help here.

1. Run /test-gmt-dev and /format in PR https://github.com/seisman/pygmt/pull/1 to see if slash commands works for external contributors
2. Fork my repository into your own account, make minor changes that fail the style checks, submit a PR to my fork, then run /test-gmt-dev and /format in your PRs.

[weiji14]

1. Run /test-gmt-dev and /format in PR https://github.com/seisman/pygmt/pull/1 to see if slash commands works for external contributors

Done at https://github.com/seisman/pygmt/pull/1#issuecomment-1876078715. Seems to work seisman@4c0534c?

2. Fork my repository into your own account, make minor changes that fail the style checks, submit a PR to my fork, then run /test-gmt-dev and /format in your PRs.

Opened at https://github.com/seisman/pygmt/pull/2. The format commands errors with:

remote: Permission to weiji14/pygmt.git denied to github-actions[bot].
fatal: unable to access 'https://github.com/weiji14/pygmt/': The requested URL returned error: 403
Error: Process completed with exit code 128.

[weiji14]

In the GitHub Actions workflow, you could try setting the content permissions (https://docs.github.com/en/actions/using-jobs/assigning-permissions-to-jobs):

    permissions:
      contents: write  # to git push added/changed files

But I'm not sure if it'll really work. I've tried using GITHUB_TOKEN last September at CryoInTheCloud/hub-image#88 (comment), and getting things to work from forks is still an issue.

[seisman]

In the GitHub Actions workflow, you could try setting the content permissions (https://docs.github.com/en/actions/using-jobs/assigning-permissions-to-jobs):

    permissions:
      contents: write  # to git push added/changed files

But I'm not sure if it'll really work. I've tried using GITHUB_TOKEN last September at CryoInTheCloud/hub-image#88 (comment), and getting things to work from forks is still an issue.

I don't think it will work, because the default permissions are:

GITHUB_TOKEN Permissions
  Actions: write
  Checks: write
  Contents: write
  Deployments: write
  Discussions: write
  Issues: write
  Metadata: read
  Packages: write
  Pages: write
  PullRequests: write
  RepositoryProjects: write
  SecurityEvents: write
  Statuses: write

Anyway, at least we can remove PAT from .github/workflows/slash-command-dispatch.yml and .github/workflows/ci_tests_dev.yaml.