Support cargo:rustc-link-arg=FLAG instruction in build script runner

[ddeville]

Cargo 1.50 added support for cargo:rustc-link-arg and cargo:rust-link-arg-bins instructions in build script output (rustc-cdylib-link-arg has been supported since 1.35 and cargo:rustc-link-arg-bin was later added in 1.54).

The build script runner currently prints a warning when rustc-cdylib-link-arg is used but entirely ignores the other ones (likely setting them as BuildScriptOutput::DepEnv since this is the default branch).

This patch adds support for the cargo:rustc-link-arg instruction and prints an warning when cargo:rust-link-arg-bins and cargo:rust-link-arg-bin are used, just like rustc-cdylib-link-arg.

Note that cargo:rustc-link-arg is important because cargo errors when cargo:rustc-flags is used for anything that isn't a -l or -L flag (nb: this is enforced by cargo so it'd work with Bazel). There are cases (such as linking a -weak_framework or -weak_library on macos) that cannot use cargo:rustc-link-lib and passing a custom -Clink-arg is required)

I think that we should also support cargo:rust-link-arg-bins, rustc-cdylib-link-arg and cargo:rustc-link-arg-bin but that's a bigger task since it requires knowing the crate type the build output will apply to. I've made issue #1062 for it.

Ref: https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script

[illicitonion]

Thanks!