Build exceeds runner storage limits

[victorlin]

#165 [linux/arm64 final 22/22] RUN chown nextstrain:nextstrain /nextstrain/build
#165 DONE 0.2s
#166 exporting to image
#166 exporting layers
…
System.IO.IOException: No space left on device

This started happening 3 weeks ago with #275's PR-triggered run.

I don't think it's caused by changes from #275 because this is happening before the call to actions/upload-artifact.

The ubuntu-latest runner provides 14 GB of storage. The Docker build is multi-platform with 3 stages, so it isn't too surprising that we've hit the limit. This has been the case for a while, but it's possible that the builds were already close to the limit, and bundled software tools have recently increased in size with latest updates.

Possible solutions

1. Build image with one job per platform
2. Other smaller scale changes to reduce storage usage
3. Use larger runners ($)

[victorlin]

A good way to address this would be to split the load across multiple jobs, i.e. one job per platform. This is also a solution to #234. I pushed ff72488 to test this out. Results look promising, so I'll turn it into a proper PR.