Add support for build script [env_flags] sub-target

[yxdai-nju]

Summary

This PR allows Reindeer to generate rules that utilize environment variable outputs from build scripts.

Relates to facebook/buck2#929.

Changes

• Add support for the [env_flags] sub-target on buildscript_run

For example, for the problematic mime_guess crate, Reindeer can now generate:

# BUCK
cargo.rust_library(
    name = "mime_guess-2.0.5",
    crate_root = "mime_guess-2.0.5.crate/src/lib.rs",
    env_flags = ["@$(location :mime_guess-2.0.5-build-script-run[env_flags])"],
    rustc_flags = ["@$(location :mime_guess-2.0.5-build-script-run[rustc_flags])"],
    # ...
)

This eliminates the need for manual environment variable fixups for crates that use cargo:rustc-env.

Testing

A demonstration repository verifies this works for mime_guess and ring crates, both have cargo:rustc-env outputs from build scripts.