rust: set ANDROID_RUST_VERSION based on prebuilts

ANDROID_RUST_VERSION is always set to the default Rust version, rather
than than respecting the RUST_PREBUILTS_VERSION setting. This is not the
correct behavior, this CL resolves the issue.

Test: m nothing
Change-Id: I3f8a2fc3590b7313286278b77534bb669cc693d6