Update CI with Node16-compatible actions

[tnull]

Github recently announced the deprecation of Node12-based actions.

We should upgrade all actions of the CI to stop them complaining.

Currently blocked on actions-rs/toolchain#220.

[TheBlueMatt]

WTH? Why is there any node.js in any of our CI? That's bonkers...

Also, I really love that issue:

I would argue that this is why we need repos like this to be run as DAOs.

[arik-so]

There's also the whole set-output thing that's Github's complaining about that doesn't appear like it should be an issue: https://github.com/actions/toolkit/blob/819157bf872a49cfcc085190da73894e7091c83c/packages/core/src/core.ts#L192

It also seems entirely due to actions-rs/toolchain, which is calling actions/core's setOutput command in multiple places in this file, starting here: https://github.com/actions-rs/toolchain/blob/88dc2356392166efad76775c878094f4e83ff746/src/versions.ts#L72

[arik-so]

Considering actions-rs/toolchain specifies the @actions/core dependency as ^1.2.6, and the latest version of @actions/core is 1.10.0, I don't quite see why we're getting the set-output error message from Github all the time.

[arik-so]

We could temporarily just fork actions-rs/toolkit and run that with PR 220 applied? It doesn't look like svartalf is coming online anytime soon.