Rollup merge of #126572 - onur-ozkan:channel-problem, r=clubby789

override user defined channel when using precompiled rustc

We need to override `rust.channel` if it's manually specified when using the CI rustc. This is because if the compiler uses a different channel than the one specified in config.toml, tests may fail due to using a different channel than the one used by the compiler during tests.

For more context, see rust-lang/rust#122709 (comment).