The rules should test a min Rust version in CI

[UebelAndre]

rules_rust has some Rust targets which are exposed to users, meaning, built using their toolchains. Commonly, the toolchains come from rust_repositories where users may explicitly set a rust version. eg:

rust_repositories(
    version = "1.53.0",
    edition = "2018",
)

The rules try to default version to the latest released version of Rust but in cases where users have pinned it, they may find themselves in a situation where they consume a new release of the rules but can no longer build due to a change in one of these exposed targets which brings in new functionality not in the pinned version or Rust.

To name a few exposed targets:

• @rules_rust//cargo/cargo_build_script_runner
• @rules_rust//tools/rustfmt
• @rules_rust//tools/rust_analyzer
• @rules_rust//util/launcher
• @rules_rust//util/dir_zipper

I think having a min-tested version of Rust for the rules would help catch these issues and should be built into the upcoming release model (#415).