CI: Use safer GITHUB_OUTPUT

[SoniEx2]

apparently this is a thing https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

tho they did go back on it https://github.blog/changelog/2023-07-24-github-actions-update-on-save-state-and-set-output-commands/

but it seems like a good idea to switch anyway

[sbc100]

Is there some way we can test this or confirm its correct? Or do we just need to wait for a release?

[SoniEx2]

the wasi job uploads artifacts, which is how we came across the deprecation warnings. and it seems to be still working. you can see them here: https://github.com/WebAssembly/wabt/actions/runs/13314673812/attempts/1?pr=2540

[SoniEx2]

(we hope that s390x failure was some sort of spurious bitflip...)