rust: always update Cargo.lock.in

rust: always update Cargo.lock.in

On a normal project where the Cargo.lock is checked in, it would be
normal to see an updated Cargo.lock in git status and the like. As we
use autoconf to generate this file, we should just copy it back to the
input file so we get the same convenience of seeing when it is
updated, which usually means it needs to be checked in.

However, to satisfy "make distcheck", only copy it if the input
template exists, if the input template does not exist we are in an out
of tree build.