Use "nightly" binaries for MicroHs CI

[sol]

This reduces the build time by 2 minutes from over 5m down to 1m 26s.

Please wait with merging this until the related changes are upstream.

[sol]

From what I understand @augustss (please correct me if I got this wrong) isn't eager to provide platform dependent binaries from upstream and instead wants to go for a cross platform distribution approach. I think this makes a lot of sense for end users.

For CI specifically, I still can't justify not to shave off several minutes of build time when it's both easily possible and when there are no apparent downsides to this approach.

So for the time being I'm going to provide nightly binaries specifically for CI use, at least until we converge on a better option.

@MaximilianAlgehed if you want to dog feed this, then this is ready to merge, but please squash the commits.

[sol]

I think technically the -r wouldn't be necessary anymore, as dependencies are provided by the nightly.

[augustss]

I intend to provide a fastinstall target in the Makefile that installs in about 10s, but does not use any platform specific binaries.