New nightlies are at risk of tripping File too big

[kinnison]

As seen in #1970 (comment) we are now generating nightlies which have 170M rlibs in.

Our multithreaded unpacker assumes each thread won't deal with files > 100M and thus limits its RAM usage by number of threads. We now seem to have files much larger than that and so we might need to up the limit at https://github.com/rust-lang/rustup.rs/blob/master/src/dist/component/package.rs#L280 and check associated numbers to re-trim things.

/cc @rbtcollins Opinions?