Use method core.exportVariable instead of core.set-output

[Nergie]

Summary

Fix #141

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[CLAassistant]

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[seratch]

Looks good to me. Using core.exportVariable looks the right way to go https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#example-creating-an-annotation-for-an-error

[flaxel]

You don't need to change the method. Currently you write to GITHUB_ENV but the previous implementation wrote to GITHUB_OUTPUT. So it's only necessary to bump the action dependency.

[seratch]

@flaxel Thanks for sharing this and this is great to hear! We can upgrade the version instead. If @Nergie is still interested in revising this PR, we will wait for it. Otherwise, I will come up with a new PR, which just upgrades the module.

[Nergie]

Thank @flaxel and @seratch. We don't need to bump the dependencies or change the method to export variables.
@actions/core 1.10.0 fixes the issue. See release notes.
I'll close this PR and the issue #141