Allow the compute-sdk tests to run without building locally

[elliottt]

This change allows us to run node .github/actions/compute-sdk-test/main.js without first running npm build in that directory. We still need to run npm install there, but this simplifies iteration when working locally.

I also updated the install-rust action to node16 while I was at it.

[jameysharp]

I haven't really reviewed this yet, but a quick thought regarding the install-rust action specifically: it's almost empty now, it should probably be a shell fragment in a composite action instead, or even just be inlined into the only place it's used in the main workflow.

[elliottt]

I haven't really reviewed this yet, but a quick thought regarding the install-rust action specifically: it's almost empty now, it should probably be a shell fragment in a composite action instead, or even just be inlined into the only place it's used in the main workflow.

Seems reasonable to me! I switched it over to a composite action in the third commit.

[jameysharp]

Oh yeah, this reminds me: I broke the "toolchain" input when I switched this over to just reading rust-toolchain.toml. That input no longer controls the toolchain that's getting installed. So this test doesn't do what any reasonable person would think it's doing. I definitely agree this commit accurately translates main.js as it stands now into a composite action though!

[elliottt]

We should figure out if we'd like to keep this. It seems nice to have a reusable place to setup the environment variables we're relying on, but the action name seems a bit misleading now :)

[jameysharp]

rustup show does install the toolchain though, as a side-effect. 😆 Maybe I should have commented to that effect when I made that change... It just has the nice bonus of recording in the CI log exactly what toolchain it ran with.