re-add CI job to test Cargo.toml rust-version

[Skgland]

was removed in #2393 instead of being updated, see

• Improve use of unsafe Rust in arena.rs (#2391) #2393 (comment) and
• 79bc2d9#commitcomment-141790142

[Skgland]

Oh, looks like I missed #2403.
Well I will try and see whats wrong with the wasm build, if I can figure it out I will include that fix here, the CI log looks unhelpful.

[Skgland]

Locally I can successfully compile it under WSL2 running Ubuntu.
Though I saw it use a lot of RAM, around 13.3 GB I believe is the highest I saw, which is probably too much for the CI Runner.

[Skgland]

Asciinema Recording: https://asciinema.org/a/J6luDBqPiwsnXxWpri3hFECNC

The highest RAM usage appears to be around 11:41 with 15.2 GB Total System usage and 13.6 GB RES / 16.7 GB VIRT Memory usage for the rustc tasks.

[infogulch]

Previous discussion about compilation memory usage: #2139

We could enable swap, there's a GH action that does this: https://github.com/actionhippie/swap-space This sounds like it would be bad for compile times, but considering that memory usage spikes right at the end it could be tolerable.

[Skgland]

Looks like increasing the swapfile size from the apparently default 4GB to 10GB worked.
And it isn't even the slowest job.