crate_universe: `generate_cargo_toml_env_vars=True` doesn't propagate CARGO_PKG_* env to build script execution (built crate, rav1e)

## Context

Follow-up to #3527. With `generate_cargo_toml_env_vars = True` set on the crate_universe extension, I expected `CARGO_PKG_AUTHORS` (and friends) to be available at **build script execution** time — but it's only available at **build script compilation** time. The result: `rav1e`'s `build.rs` (which uses the [`built`](https://crates.io/crates/built) crate) panics with:

```
thread 'main' panicked at external/crate_index__built-0.7.5/src/environment.rs:53:9:
Missing expected environment variable "CARGO_PKG_AUTHORS"
```

## Root cause

In `crate_universe/src/rendering.rs:380` (0.70.0):

```rust
if self.config.generate_cargo_toml_env_vars {
krate.common_attrs.rustc_env_files.insert(":cargo_toml_env_vars".to_owned(), None);
if let Some(ref mut build_script_attrs) = &mut krate.build_script_attrs {
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); // <-- this line is dead
}
}
```

The `build_script_env_files` insertion is supposed to wire `cargo_toml_env_vars` into the cargo_build_script's *execution* environment. But the `CargoBuildScript` starlark struct in `crate_universe/src/utils/starlark.rs:95` has no `build_script_env_files` field — only `rustc_env_files` is serialized. So the insertion never reaches the rendered BUILD.bazel.

```rust
#[derive(Debug, Serialize)]
#[serde(rename = \"cargo_build_script\")]
pub(crate) struct CargoBuildScript {
...
#[serde(skip_serializing_if = \"SelectSet::is_empty\")]
pub(crate) rustc_env_files: SelectSet, // present
// missing: build_script_env_files
...
}
```

Compare with `cargo/private/cargo_build_script_wrapper.bzl` which does accept `build_script_env_files`.

## Reproducer

Add `rav1e = \"0.7\"` to a `Cargo.toml`, render with crate_universe (default `generate_cargo_toml_env_vars = True`), try to build it. Build script panics as above.

## Proposed fix

Add `build_script_env_files: SelectSet` to the `CargoBuildScript` struct, populate it from `attrs.build_script_env_files` in `make_cargo_build_script`, and ensure the template emits it.

We carry a workaround patch that hardcodes empty defaults for `CARGO_PKG_AUTHORS/DESCRIPTION/HOMEPAGE/LICENSE/REPOSITORY` in `cargo/private/cargo_build_script.bzl` — not great because it doesn't use the actual Cargo.toml values. The proper fix would let the existing `cargo_toml_env_vars` rule do its job.

## Versions

- rules_rust: 0.70.0
- bazel: 7.7.1