Update the rust toolchain nightly version.
# Before upgrading check that everything is available on all tier1 targets here:
# https://rust-lang.github.io/rustup-components-history
[toolchain]
-channel = "nightly-2021-09-15"
+channel = "nightly-2021-09-16"
components = [ "rust-src", "rustfmt" ]
targets = [ "thumbv7em-none-eabi", "thumbv7m-none-eabi", "thumbv6m-none-eabi", "thumbv7em-none-eabihf", "wasm32-unknown-unknown" ]
