Rollup merge of #144159 - ognevny:opt-dist-build-dir, r=Kobzol

opt-dist: change build_dir field to be an actual build dir

make it configurable so users can set build.build-dir option in bootstrap.toml

r? Kobzol

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

Author: jhpratt