Make message about rust-toolchain.toml a warning

[joshka]

Problem you are trying to solve

In our Github CI for Ratatui, we have a setup which installs rust beta using dtolnay/rust-toolchain, which eventually delegates to rustup. We anticipated that this would cause cargo to run using the beta toolchain, but missed that rust-toolchain overrides this. There is an existing message that highlights this problem in rustup, but I think this should likely be written as a warning level rather than an info level message:

https://github.com/ratatui/ratatui/actions/runs/18035682655/job/51321994095#step:3:84

info: note that the toolchain 'stable-x86_64-unknown-linux-gnu' is currently in use (overridden by '/home/runner/work/ratatui/ratatui/rust-toolchain.toml')

To be clear, this is about making a problematic configuration simple and obvious, it's not about changing how the rustup / rust-toolchain.toml things work.

Solution you'd like

Change this log message to an warning to make it more visible in logs

Notes

No response

[djc]

Is this distinct from #4445?

[rami3l]

Is this distinct from #4445?

Yes: the title of that issue was not exact. I've updated it accordingly. Thanks!