My chroot contained rust 1.23.0 even though 1.24.0 has apparently been available for a while.  Running emerge manually finds the update:

$ sudo emerge -pv rust

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild     U  ] dev-lang/rust-1.24.0:stable/1.24::chromiumos [1.23.0:stable/1.23::chromiumos] 224,198 KiB

But update_chroot (and build_packages etc) don't.  Should the target-chromium-os-sdk-post-cross ebuild depend on a newer version?  Or is there another step I should be following to get a newer rustc version?