Bump rust-installer

This brings in rust-lang/rust-installer#123, which enables a larger
compression window (8 -> 64MB) amongst other changes to the xz
compression settings. The net effect should be smaller compressed
tarballs which will decrease bandwidth usage for
static.rust-lang.org, download times, and decompression time.

This comes at the cost of higher baseline requirements for running
rustup to use these files, which we believe should be largely acceptable
(running rustc is likely to use at least this much memory) but if we get
specific reports we may explore options to decrease impact (e.g., using
the gzip tarballs automatically in rustup).