Completion gen faster

[tmccombs]

This is an attempt to speed up CI builds, by pulling completion generation out into a separate job. I don't know if it will work though.

[tmccombs]

This might actually make it slower though, by having additional jobs.

Some other things we might be able to do:

1. Use my first commit on this branch so that we can re-use the output from the build step to generate the completions. That was my original plan, but then I realized it wont' work if for the jobs where we cross-compile to another architecture.
2. Set up caching so that if one job generates completions it can be re-used by other jobs. Would probably want to do 1 in addition to this.
3. Do this PR, but also use more caching, so that building on the completion jobs does work that benefits some of the build/test jobs.

[sharkdp]

Thank you for looking into this. I don't think the problem is severe enough to require urgent fixing - so we can take our time to find a good solution.

I'm not an expert in GitHub Actions, so I can't really help much here. I just usually try to avoid any kind of caching solutions in CI if possible, because that just introduces a whole new class of potential problems where the cache is not invalidated properly in certain scenarios. I'd rather keep CI simple, even if it takes a bit longer.