We could download everything easily (any version); if we do go the build-all route, I would suggest building all cross-target libstds at once (that would mean hardcoding deps on at least cc for both 64 and 32 bits) but I think that's preferable than 4-5 hours of rebuilding each time. Either that or download.
And so far everybody in the rust discord says this is to be expected because of unstable ABI.
Since we're mostly using rust as prerequisite for other things, maybe downloading is not a sin (we could stash the building script somewhere in case the situation changes in the future of course)