Rollup merge of #144164 - ognevny:opt-dist-stage0-root, r=Kobzol

 opt-dist: add an option for setting path to stage0 root

in MSYS2 we have problems with stage0 for *-gnullvm hosts because prebuilt dist tarballs will be
available starting from 1.90.0-beta. also this change helps to match bootstrap.toml config

r? Kobzol

try-job: dist-x86_64-msvc
try-job: dist-x86_64-linux

Author: matthiaskrgr