Split CI dist jobs to use free runners

[marcoieni]

We could split the large runners used in https://github.com/rust-lang/rust/blob/master/src/ci/github-actions/jobs.yml
I.e. job-linux-8c, job-windows-8c, job-aarch64-linux-8c and job-linux-36c-codebuild to two or more jobs that use standard runners.

We're not sure if this is possible. We want the overall duration of the autobuild to remain the same.

How:

• the jobs compile only a subset of the artifacts and uploads them (probably compressed) somewhere (github artifacts or s3?)
• another job downloads these artifacts (probably decompresses them) and uploads them in the same format as the original job