ci: test nix build on push with linux & macos machines

[vinnymeller]

wip

[Saghen]

on the other hand this isn't uploading artifacts like the others, more of just a test, so it doesn't fit super neatly into the release workflow

Yeah, I think it makes sense to run this on: push. Overall seems like a good idea!

[vinnymeller]

@Saghen would you want a separate workflow to just run the nix build then? like .github/workflows/nix-build.yaml or something?

[Saghen]

Yeah that'd be great

[Saghen]

Thank you!