… this time using a `rust-toolchain` file compatible with rustup:
https://github.com/rust-lang-nursery/rustup.rs/#the-toolchain-file

And upgrade to rustc 1.21.0-nightly (c11f689d2 2017-08-29)

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Now if both `system-rust` and `system-cargo` are set to `true` in `.servobuild`’s `[tools]` section,
and the corresponding `rustc` and `cargo` binaries are in fact rustup’s wrappers,
then rustup will use the correct version based on `rust-toolchain`.

CC #11361

Unlike #17927,
this does not make mach use rustup directly.