GHA: Replace deprecated set-output

[tim-dlang]

See https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

This should fix a warning for the nightly builds in bugzilla issue 24600.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	24600	normal	DMD nightly builds are outdated

[kinke]

Thanks!