Fix skip-ci on stable

[jimmygchen]

Issue Addressed

Fix skip-ci job triggered by pushing to branches. Also replaced the deprecated ::set-output command.

I've tested this on a push to my fork, and it evaluates null without failing:
https://github.com/jimmygchen/lighthouse/actions/runs/9457252549/job/26050717847

[michaelsproul]

As much as I hate "null" as a string, this looks like it would work

LGTM

[michaelsproul]

@Mergifyio queue

[mergify]

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[michaelsproul]

@Mergifyio requeue

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[mergify]

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