Chore: replace deprecated set-output in the deploy workflow

[katspaugh]

What it solves

set-output will be deprecated in May this year, so we need to migrate it as per https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

I've copied this portion of the code from web-core, where it has worked well for a while, so I assume it will work here as well.

[github-actions]

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Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	0	N/A

• Result: ✅ success
• Annotations: 0 total

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^{Report generated by eslint-plus-action}