Refactor `opt-dist` to simplify local building This PR refactors the `opt-dist` tool to make it easier to invoke it locally, outside of CI, and thus simplify building PGO/BOLT optimized `rustc` builds e.g. for distro maintainers. It should also make it easier to run the PGO/BOLT workflow locally e.g. to profile performance or debug issues (looking at you, https://github.com/rust-lang/rust/pull/115554).