Setup ANDROID_RUST_VERSION env variable

Some crates (i.e. rustversion) probe at build time the version of Rust
to enable features or tests. Pass the version via an environment
variable so it can be used by a patched crate.

Bug: 178357400
Test: m librustversion
Change-Id: If57a6339b71d2188c56e7dc6709075e4537403b4