Add option to ./mach bootstrap to fetch artifacts from tor-browser-build

This would be nice to have developers and CI using the same toolchains as the nightly and release builds.

Moreover, there is already a few artifacts we (more or less) fetch from tor-browser-build, such as tor-expert-bundle when building for Android or even ctor. It would be nice to centralize the fetching to in a single place in the code.

This ticket involves also modifications to tor-browser-build itself to match the toolchain names to the same names and formats expected by the ./mach.