ci: Use GITHUB_OUTPUT envvar instead of set-output command

[arunsathiya]

save-state and set-output commands used in GitHub Actions are deprecated and GitHub recommends using environment files.

This PR updates the usage of set-output to $GITHUB_OUTPUT

Instructions for envvar usage from GitHub docs:

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-output-parameter

[sdroege]

Thanks

[sdroege]

Seems like something is broken on the Windows CI, which doesn't look like your fault though.

[pbor]

Actually it seems to me like the failure is caused by the patch :)

The Windows CI uses

    outputs:
      cachekey: ${{ steps.output.outputs.cachekey }}

to lookup the cached glib binaries. With this patch, that key is not present anymore