`WorkerRunInfo` has two fields `worker` and `exe` for the persistent worker command or the
non-worker mode command respectively. This commit changes the remote persistent worker example to
use these appropriately to distinguish between worker and non-worker mode execution.

The Buck2 Rust implementation needs to use the worker command instead of the non-worker command for
remote persistent worker execution mode.

We want to use a Buck2 managed hermetic Python toolchain. However, Python binaries generated by such
a toolchain are sensitive to `PWD` and don't work in other working directories than the repository
root. Unfortunately, `genrule` changes directory and is hence incompatible with such Python
binaries. This commit defines a dedicated rule to invoke protoc without changing the working
directory.

The previous setup used Nix to provide a Python toolchain and packages in a reproducible fashion.
However, this requires dedicated remote worker images with the Nix store paths pre-populated which
complicates the setup for testing on Buck2 CI.

Using a hermetic Python toolchain avoids these issues and works on the standard remote execution
image.

Remove the old genrule targets to generate the Python gRPC/protobuf bindings.

Without Nix it is no longer required to use a custom remote worker image.

It's useful to test multiple builds in parallel to catch potential issues related parallel worker
requests. However, we also don't need to be excessive in the number of tests to not unnecessarily
waste resources.

Requires a repository secret to be set up for the BuildBuddy API key named `BUILDBUDDY_API_KEY`.