Rollup merge of rust-lang#63984 - cuviper:rust-installer-mem, r=Mark-Simulacrum

Update rust-installer to limit memory use

See rust-lang/rust-installer#98 -- on a many-core machine, the xz memory for so many threads is more than a 32-bit process can handle. The xz stream is now limited to 8 threads.

r? @alexcrichton

Author: Centril