proto: Pass in the plugin as tools to preserve runfiles (#4514)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

The proto plugin should preserve runfiles so that bazel targets that
rely on external data can find them. It does this by by running the
plugin as a tool, which is correct because plugins are expected to be
built and run using the exec configuration.

**Which issues(s) does this PR fix?**

Fixes #4513