'How can I make GitHub Actions work with PRs made by external contributors?

My GitHub actions depend on some GitHub secrets that I set in my repo:

CI works when I push a commit:

But it fails when an external contributor makes a PR from their forked version of the repo:

The execution fails because of this error:

Error: Please set MNEMONIC as an env variable

But the MNEOMNIC secret is there. Why doesn't it work when the GitHub Action is triggered by a PR made by an external contributor?

Sources

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