rust: Add support for CARGO_PKG_NAME

Technically cargo package names and crate names can be different, but
this is rare and usually only a matter of presentation. Adding basic
support in this fashion lets several crates build without adding
significant machinery.

Bug: 276464273
Test: Build a crate which uses this environment variable
Change-Id: I4f4c8c9e277789de7a63be2457f00090e8c3e973