rust: Add support to emit certain Cargo env vars.

Some crates expect Cargo to provide certain environment variables. This
CL adds a compatability flag that emulates the behavior of Cargo by
setting these environment variables when building.

Bug: 171011485
Test: New soong tests pass
Test: quiche no longer requires patch removing CARGO_PKG_VERSION
Change-Id: I4c95c284846f6075428c6f61fe8c260f2e35fbd9