Use the current rustc version as the default RustTarget

[pvdrz]

How about changing RustTarget::default to return the version of the rust compiler in use when the __cli feature is not enabled?

Originally posted by @glandium in #3052

[decathorpe]

I wonder why only "when the __cli feature is not enabled"?

Even the CLI defaulting to a Rust version that is potentially newer than the actually available Rust compiler isn't great. It's now biting us on RHEL / EPEL 9 (which currently ships Rust 1.79), which I didn't catch during testing that I did before pushing the 0.71 update.