Check for nightly toolchain when enabling split debug info. (#4186)

This fixes a problem pointed out by

https://github.com/bazelbuild/rules_rust/pull/4092#issuecomment-5136906807.

`-Zsplit-dwarf-out-dir` is only available on nightly, so we have to gate
split debug info on it. Now, if the user requests `--fission` while
using a non-nightly Rust toolchain, the build will fail. To avoid
breaking users relying on the previous behavior, a
`skip_fission_for_rust` flag has been added. Users can add it to their
bazelrc file to explicitly signal that they're okay with Rust compiles
not producing split debug info, even if requested in the command line.

Assisted-by: Gemini