Attempt to split up CI build steps into separate jobs

[mitchmindtree]

Not sure if this will work just yet, but hoping this splits up the
build CI job to execute jobs in parallel, synchronising where
necessary using the <job_id>.needs.<other_job_id> feature.

Hopefully this cuts down on waiting time and makes the results a little
more readible from within the PRs.

Closes #772.

[mitchmindtree]

OK after lots of fiddling around I think this might be ready review!

Apart from splitting up the build job into its parts, this also moves the clippy check from a separate file into the main CI workflow and adds it to the list of jobs that must be successful before publishing can go ahead.

Here's what the workflow job graph looks like now:

The last commit took about 5mins altogether, but I think this is just because it was hitting the Rust cache for every job. When not hitting the cache, it seems to take 10-12 minutes (this is just from eyeballing the actions times here). Either way, I think this is a bit better than the 17-20mins we've been getting when not hitting the cache with our current long sequential build job.

Oh yeah, I also haven't re-added this as I'm unsure what we're using it for:

      - name: Set git config
        run: |
          git config --global core.bigfilethreshold 500m

Do we still need it?

[Voxelot]

It looks like some job requirements are blocking this?

[mitchmindtree]

@Voxelot I think it might only be blocked on approval:

I don't think those old CI checks need to pass, though I could be wrong!

[Voxelot]

I think it's possible to merge without CI passing, so we should probably root out why these 2 jobs are stuck? Otherwise we may never know when CI fully succeeded or failed.

[mitchmindtree]

I think they're not running because they're removed as a part of this PR in favour of the new jobs (this PR only touches the github workflow and doesn't touch any code). Is that what you mean?

[Voxelot]

I guess what I'm wondering is why the removed jobs are still expected. Is this because the PR isn't merged to master yet? Or is there some kind of config setting on the repo expecting these jobs?

[adlerjohn]

Fixed the repo config to not require jobs, which had the old name