This should be slightly more correct, and matches the implementation in other programming languages:
- [Python's `os.environ`](https://github.com/python/cpython/blob/v3.12.3/Modules/posixmodule.c#L1562-L1566).
- [Swift's `Darwin.environ`](https://github.com/apple/swift-corelibs-foundation/blob/swift-5.10-RELEASE/CoreFoundation/Base.subproj/CFPlatform.c#L1811-L1812), though that library is bundled on the system, so they can change it if they want.
- [Dart/Flutter](https://github.com/dart-lang/sdk/blob/3.4.0/runtime/bin/platform_macos.cc#L205-L234), doesn't support environment variables on iOS.
- Node seems to not be entirely consistent with it:
  - [`process.c`](https://github.com/nodejs/node/blob/v22.1.0/deps/uv/src/unix/process.c#L38).
  - [`unix/core.c`](https://github.com/nodejs/node/blob/v22.1.0/deps/uv/src/unix/core.c#L59).
- [.NET/Xamarin](https://github.com/dotnet/runtime/blob/v8.0.5/src/native/libs/configure.cmake#L1099-L1106).
- [OpenJDK](https://github.com/openjdk/jdk/blob/jdk-23%2B22/src/java.base/unix/native/libjava/ProcessEnvironment_md.c#L31-L33).

If we're comfortable using `_NSGetEnviron` from `crt_externs.h`, there shouldn't be an issue with using these either, and then we can merge with the macOS implementation.

This also fixes two test cases on Mac Catalyst:
- `tests/ui/command/command-argv0.rs`, maybe because `[[NSProcessInfo processInfo] arguments]` somehow converts the name of the first argument?
- `tests/ui/env-funky-keys.rs` since we no longer link to Foundation.