Do not build nightly-only tools on beta/stable CI #74709
Description
Currently, we build nightly-only tools such as Miri even on beta/stable CI, only to then ignore whether they build or not. That seems like a waste. We certainly do that for the tools builders (that also run the tests); I do not know if we also do it for dist builders (that just build the tools to put them into rustup packages).
#74389 is the beginning of fixing that, but I got stuck due to not knowing enough about rustbuild.