extractable (from reproducible.Dockerfile) pinned toolchain

[spoonincode]

Our pinned toolchain is built inside of reproducible.Dockerfile and then used either in CI or through the build target of reproducible.Dockerfile as used for manual building. But if there is an error/warning/defect during the build it's not easy for leap developers to use the pinned toolchain to troubleshoot it. Leap developers may want to use the pinned toolchain otherwise too (to play around with libc++ or such).

It'd be nice if the pinned toolchain was extractable from the reproducible.Dockerfile builder stage.

I attempted this in the repro_extractable_wip branch but unfortunately while it worked great on Arch, on Ubuntu setting a CMAKE_TOOLCHAIN_FILE seems to be preventing cmake from finding libraries like GMP or libz even. Not sure what's going on just yet.