…non-persistent case

… dir.

Repositories should always be registered.

from commit:
nikhilm/rustc-worker@47d2de4

The regular rust_binary() and rust_library() rules require a worker
toolchain, even if it is a dummy one. This prevents us from using
these rules to build the worker binary, as it leads to a cyclic
dependency.

Distributing an external binary is not ideal - it's hard to verify
what it's doing, and awkward to add on new platforms. To work around
this, we invoke cargo directly, using the configured toolchain. This
causes cargo to fetch the dependencies directly, and
build the worker binary which we can then use for future compilation.

Windows builds without a sandbox, and the calling Bazel workspace
is higher up in the path, so if it has a Cargo workspace it breaks
the build.

This is an ugly hack, and hopefully can be done in a nicer way.

Otherwise with a local executor (eg Windows), we end up placing
build products into the source tree.

Should fix bazelbuild#517 (comment)

This lets us enable/disable the worker from the command line or
.bazelrc, and allows us to avoid a separate dummy toolchain.