rust: set STD_ENV_ARCH

In 1.47.0, std::env requires the STD_ENV_ARCH environment variable to be
set to the target architecture [1].

[1] https://github.com/rust-lang/rust/blob/8e8939b8045b7af6076fb718e2e298844aaf4650/library/std/src/env.rs#L866

Bug: 171280890
Test: cd external/rust; mm
Change-Id: If4ddf4b9bad2a1a7518a358a5fb597170a97c67e