Fix cargo_build_script env vars from cargo_toml_env_vars for build script execution

Problem:
Build scripts for several crates were failing when using crate_universe
with generate_cargo_toml_env_vars=True (the default). The failures
occurred because CARGO_PKG_* environment variables extracted from
Cargo.toml were not being passed to build script execution.

Affected crates included:
- rav1e 0.7.1: Its build script depends on the built crate (0.7.7),
  which calls std::env::var("CARGO_PKG_AUTHORS") at runtime and panics
  if the variable is missing.
- typenum 1.19.0: Build script failed with "Wrong environment file
  format" because CARGO_PKG_DESCRIPTION contained embedded newlines
  escaped with backslashes.

Root causes:

1. Missing build_script_env_files in CargoBuildScript:
   The render_one_build_file() function in rendering.rs correctly
   populates build_script_attrs.build_script_env_files with the
   ":cargo_toml_env_vars" target. However, the CargoBuildScript struct
   in starlark.rs lacked a build_script_env_files field, so this data
   was never serialized to the generated BUILD.bazel files.

   Note: rustc_env_files passes env vars to rustc for compile-time
   env!() macro expansion. build_script_env_files passes env vars to
   the build script binary at runtime for std::env::var() access.

2. Line continuation parsing in cargo_build_script_runner:
   The cargo_toml_variable_extractor correctly escapes newlines in
   values using backslash continuations (e.g., "value\\nmore").
   However, the build script runner's env file parser split on '\n'
   without handling continuations, causing lines like "    compile
   time." to fail the KEY=value format check.

Fix:

1. Add build_script_env_files field to CargoBuildScript struct and
   populate it from build_script_attrs in make_cargo_build_script().

2. Update the env file parser to accumulate lines ending with '\'
   before splitting on '=' to extract the key-value pair.

Fixes build failures for crates whose build scripts read CARGO_PKG_*
variables at runtime, including those with multiline descriptions.

Author: Adjective-Object

cargo/private/cargo_build_script_runner/bin.rs

@@ -100,12 +100,25 @@ fn run_buildrs() -> Result<(), String> {
 
     for dep_env_path in input_dep_env_paths.iter() {
         if let Ok(contents) = read_to_string(dep_env_path) {
+            // Handle line continuations: lines ending with '\' continue on the next line
+            let mut current_line = String::new();
             for line in contents.split('\n') {
-                // split on empty contents will still produce a single empty string in iterable.
-                if line.is_empty() {
+                // Skip empty lines when we're not in a continuation
+                if line.is_empty() && current_line.is_empty() {
                     continue;
                 }
-                match line.split_once('=') {
+
+                if let Some(prefix) = line.strip_suffix('\\') {
+                    // Line continues on the next line
+                    current_line.push_str(prefix);
+                    current_line.push('\n');
+                    continue;
+                }
+
+                // Complete line (either standalone or end of continuation)
+                current_line.push_str(line);
+
+                match current_line.split_once('=') {
                     Some((key, value)) => {
                         command.env(key, value.replace("${pwd}", &exec_root.to_string_lossy()));
                     }
@@ -115,6 +128,7 @@ fn run_buildrs() -> Result<(), String> {
                         )
                     }
                 }
+                current_line.clear();
             }
         } else {
             return Err("error: Dependency environment file unreadable".to_owned());

crate_universe/src/rendering.rs

@@ -641,6 +641,12 @@ impl Renderer {
                     .unwrap_or_default(),
                 platforms,
             ),
+            build_script_env_files: SelectSet::new(
+                attrs
+                    .map(|attrs| attrs.build_script_env_files.clone())
+                    .unwrap_or_default(),
+                platforms,
+            ),
             rustc_flags: SelectList::new(
                 // In most cases, warnings in 3rd party crates are not
                 // interesting as they're out of the control of consumers. The

crate_universe/src/utils/starlark.rs

@@ -98,6 +98,8 @@ pub(crate) struct CargoBuildScript {
     pub(crate) aliases: SelectDict<Label, String>,
     #[serde(skip_serializing_if = "SelectDict::is_empty")]
     pub(crate) build_script_env: SelectDict<String, String>,
+    #[serde(skip_serializing_if = "SelectSet::is_empty")]
+    pub(crate) build_script_env_files: SelectSet<String>,
     #[serde(skip_serializing_if = "Data::is_empty")]
     pub(crate) compile_data: Data,
     #[serde(skip_serializing_if = "SelectDict::is_empty")]