CI: pin GitHub Actions workflows

[ErikSchierboom]

This PR updates GitHub Actions workflows to a specific version. This ensures that the workflow will always run the same code, which makes your build stable. It will also prevent a potential security issue where a tag could be replaced by a malicious commit without consumers being aware of it.

The PR updates each non-SHA based workflow reference with the SHA of the referenced version/tag, so the current behavior should not change.

See https://exercism.org/docs/building/github/gha-best-practices#h-pin-actions-to-shas for more information.

[github-actions[bot]]

Pull Request Test Coverage Report for Build 6864591962

• 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 65.835%

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Totals
Change from base Build 6863048637:	0.0%
Covered Lines:	422
Relevant Lines:	641

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💛 - Coveralls