Please honour CARGO_NET_OFFLINE for implicit installs

Steps

1. Don't have +nightly-2023-12-19 installed
2. In any project, CARGO_NET_OFFLINE=true cargo +nightly-2023-12-19 check --locked --offline

Expected behaviour

Error message saying +nightly-2023-12-19 is not installed, mentioning CARGO_NET_OFFLINE

Actual behaviour

Starts to download the requested toolchain.

Discussion

CARGO_NET_OFFLINE is a way that cargo has provided for some time now to allow disabling network access. This can be very useful in a number of circumstances.

A user might set this because:

• The network is unavailable or very slow and they don't want to wait for it to time out
• The network is very expensive, or rationed, so they need to control downloads
• They want to prevent downloads for security or software-supply-chain reasons

See also #4264. I'm hoping that honouring CARGO_NET_OFFLINE will be uncontroversial, and won't be blocked on those wider discussions.

Reproduce the command from the issue with an uninstalled nightly toolchain and compare the offline behavior with the normal implicit-install path. Trace the rustup command entry point that handles the requested toolchain and determine where CARGO_NET_OFFLINE should prevent downloading. Done means the command reports that the toolchain is not installed, mentions CARGO_NET_OFFLINE, and does not start a download.