Make the default behaviour of `cargo build` match the documentation

Make the default behaviour of `cargo build` match the documentation

cargo build --help says:

    --all-targets                Build all targets (lib and bin targets by default)

but the old default behaviour was actually doing --all-targets. This meant that
--avoid-dev-deps was only effectively if one gave --lib --bins explicitly. With
this commit, `cargo build --avoid-dev-deps` with no other flags, will build lib
and bins and avoid installing dev-dependencies.