ci: fix GitHub Actions workflow to run on contributors' pull requests

[andreoliwa]

Proposed changes

1. GitHub Actions workflow is not running on contributors' pull requests: https://github.com/andreoliwa/nitpick/pull/577
2. Run for both push and pull request but try to skip duplicates with https://github.com/marketplace/actions/skip-duplicate-actions

Checklist

• [x] Read the contribution guidelines
• [x] Run make locally before pushing commits
• Add tests for the relevant parts:
  • API
  • CLI
  • flake8 plugin (normal mode)
  • flake8 plugin (offline mode)
• Write documentation when there's a new API or functionality

[coveralls]

Pull Request Test Coverage Report for Build 5112842033

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.347%

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Totals
Change from base Build 5112709368:	0.0%
Covered Lines:	1992
Relevant Lines:	2051

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💛 - Coveralls

[github-actions[bot]]

:tada: This PR is included in version 0.33.2 :tada:

The release is available on:

• v0.33.2
• GitHub release

Your semantic-release bot :package::rocket: