sysroot is not properly populated by rustChannelOfTargets

[nagisa]

Running

$ nix-shell -p 'rustChannelOfTargets "nightly" null ["i686-unknown-linux-gnu" "x86_64-unknown-linux-musl"]'

will put the standard library for the specified targets in their own little unique directories where rustc has no chance of finding them. The standard libraries should all end up in the same lib/rustlib as the native libstd.