We need to build in release mode so targets are cacheable.
GH has a limit of 400MB per cache blob and we're at 750MB with
the debug build. Slim it down a bit.
The cache wants to hash the Cargo.lock file so I've removed that
from the ignore file and added it to the repo. This might cause
an error for users checking out the repo after this change is
pushed; removing the local Cargo.lock should resolve that.
