crate_universe: emit build_script_env_files on rendered cargo_build_script targets

[ashi009]

Fixes #4037. Reproducer: https://github.com/ashi009/rules-rust-4037-repro.

Problem

#3632 added a build_script_env_files attribute to cargo_build_script, threaded it through BuildScriptAttributes, and inserted ":cargo_toml_env_vars" into it when generate_cargo_toml_env_vars = True:

rules_rust/crate_universe/src/rendering.rs

Lines 386 to 394 in 56176ea

	build_script_attrs
	.rustc_env_files
	.insert(":cargo_toml_env_vars".to_owned(), None);
	build_script_attrs
	.build_script_env_files
	.insert(":cargo_toml_env_vars".to_owned(), None);
	}
	}

But the CargoBuildScript starlark struct in crate_universe/src/utils/starlark.rs — the one whose #[derive(Serialize)] emits cargo_build_script(...) into the rendered BUILD.bazel — was not updated to include a build_script_env_files field. Serde drops the value on the way out, so the rendered output looks like:

cargo_build_script(
    name = "_bs",
    ...
    rustc_env_files = [":cargo_toml_env_vars"],
    # build_script_env_files = [":cargo_toml_env_vars"]  ← missing
    ...
)

rustc_env_files sets env when compiling build.rs (so env!(...) works). build_script_env_files sets env when executing the compiled binary (so env::var(...) works). The built crate uses env::var("CARGO_PKG_AUTHORS") at execution time, so anything depending on built (e.g. rav1e) panics:

thread 'main' panicked at .../built-0.7.7/src/environment.rs:54:9:
Missing expected environment variable "CARGO_PKG_AUTHORS"

#3632's regression test (test_data/build_script_env_files/) exercises a hand-written cargo_build_script(build_script_env_files = [...]), which works fine because the user writes the field directly. The crate_universe rendering path was never covered, so the gap went unnoticed.

Fix

Add build_script_env_files to CargoBuildScript and populate it from attrs.build_script_env_files in make_cargo_build_script, mirroring the existing rustc_env_files plumbing. No new attributes — the rule, macro, and context struct already support it.

Verification

Repro: https://github.com/ashi009/rules-rust-4037-repro (tiny workspace with rav1e = "0.7.1").

• Without this PR: bazel build @crates//:rav1e panics with the CARGO_PKG_AUTHORS error above.
• With this PR (cargo-bazel built from this branch via CARGO_BAZEL_GENERATOR_URL): build succeeds. Generated BUILD.rav1e-0.7.1.bazel emits build_script_env_files = [":cargo_toml_env_vars"] on the cargo_build_script target.

Tested against Bazel 7.7.1, macOS arm64.