cargo metadata resolves all dependencies even ones that are disabled (known as weak dependencies)
which cargo build doesn't do (see issue rust-lang/cargo#10801). These unresolved dependencies require
network access to load their package metadata.
As the `--frozen` flag itself blocks network access (in addition to blocking changes to the Cargo.lock)
too, we need to remove both the `--frozen` and `--offline` flags. Doing this allows cargo the change the
`Cargo.lock` though, which is undesirable (as we're in the process of building the same dependencies that
rely on the `Cargo.lock` in the first place).