Make message about rust-toolchain.toml a warning

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

Search the rustup source for the exact message about the toolchain being overridden by rust-toolchain.toml, using the linked GitHub Actions log as the expected current output. Change only its severity and run the relevant rustup checks to confirm the same configuration now appears as a warning without changing toolchain behavior.