ci: split CI into separate moldy jobs

[a10y]

• use mold linker for building Rust code
• Splits into several parallel jobs
• disables one test that was taking ~7 minutes with miri, cutting the miri time down considerably

Shaves ~40% off of CI times

[robert3005]

weirdly the command pyvortex runs also does this

[danking]

yeah make -C docs html builds all the docs, including the Rust docs. We could add a different target like make -C docs full-html and use that in docs.yml.

[robert3005]

You need to run cleanup as the first thing in the job, otherwise it evicts installed binaries

[robert3005]

But also maybe we don't need it that much if we have separate jobs?

[robert3005]

Ok @a10y I think we should change the cleanup job if we are splitting things, it's now taking close to 50% of the time. for some of them now. Could probably trim down the list. I went full hog trying to make sure we don't have to do a cleanup midway through

[a10y]

Ok I've removed usage of cleanup action from all steps, and have added a new custom action to download a flatc compiler so that we can check all generated files at CI time

[robert3005]

looks like you do need some level of cleanup of hosted tools otherwise our builds step runs out of disk

[a10y]

It was node_modules that was slow to delete. Cleanup times went from 3minutes -> ~10 seconds now