fix: use the CRAN suggesting SystemRequirements field

[eitsupi]

From https://cran.r-project.org/web/packages/using_rust.html

The package should declare
SystemRequirements: Cargo (Rust's package manager), rustc

[JosiahParry]

From Using Rust in CRAN Packages

"SystemRequirements: Cargo (Rust's package manager), rustc"

This update is required. Thanks!

[JosiahParry]

@eitsupi can you use styler::style_package() to fix the linter? @Ilia-Kosenkov can you review this PR? It's simple enough and addresses the requirements in https://cran.r-project.org/web/packages/using_rust.html

[eitsupi]

@eitsupi can you use styler::style_package() to fix the linter?

The error in CI has nothing to do with this change.
The part the linter is complaining about has already been fixed in the main branch by #309

[Ilia-Kosenkov]

Do a git merge main to pull in lintr fixes.