CI - Use rust-toolchain-file everywhere

[bfops]

Description of Changes

We were using rust-toolchain in some places, and rust-toolchain-file in others. I think Rust released a new version, which made the rust-toolchain parts break with:

info: downloading component 'rustfmt'
info: removing previous version of component 'rust-src'
info: rolling back changes
error: could not rename component file from '/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/term/terminfo/searcher' to '/root/.rustup/tmp/a4eo07uz83vsyfhk_dir/bk': Invalid cross-device link (os error 18)
Error: Process completed with exit code 1.

(Separately, this breakage is confusing.. we'll probably run into this again when we roll forward our rust version..)

API and ABI breaking changes

None. CI-only change.

Expected complexity level and risk

1

Testing

• CI passes
• There are no more instances of rust-toolchain:

$ grep -rIP 'rust-toolchain(?!-file)' .github/workflows
.github/CODEOWNERS:/rust-toolchain.toml @cloutiertyler

(on master, this finds the instances we changed)