Fix bug with rustup toolchain overrides causing redundant channel update syncing

[kay]

The issue is that cross is trying to call rustup toolchain add XX on every run when toolchain overrides are in use.

Replication & more detail on the issue is given in this repo: https://github.com/kay/cross-toolchain-override-bug

This uses a rust-toolchain file to override you can also replicate using rustup override set 1.45.1 as described here

Going by the latest rustup source, this is the relevant code that enumerates the various annotations outputted for toolchains: and it looks like (default) and (override) are the only ones that may be output for running rustup toolchain list as cross -V would.

[reitermarkus]

bors r+

[bors]

Build succeeded:

• rust-embedded.cross
• shellcheck