Insufficient test isolation for `/etc/rustup/settings.toml` files

[rbtcollins]

Problem
When we added the fallback settings capability, we failed to make it properly isolated for testing :(. As such, an /etc/rustup/settings.toml file that specifies stable as the toolchain will cause update (soon to be update_once in cli-exact.rs to fail, because the default toolchain will be set before the initial update.

Since we need this to be per-test, we probably want to allow an env variable to override the path to it, then set that up in the mock test support glue.