refactor: use pre-built protoc binary

[alexeagle]

Saves a bunch of time in analysis and execution when there is a cache miss on protoc, as well as avoiding a bunch of gcc logspam.

For example a recent commit on master took 3m 54s to
Run bazel run @com_github_bazelbuild_buildtools//:buildifier

https://github.com/buildbarn/bb-storage/actions/runs/15302599382/job/43046869873

and printed 256 lines of log output such as

bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/protobuf+/src/google/protobuf/io/_virtual_includes/printer/google/protobuf/io/printer.h:127:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
  127 |     for (int i = 0; i < path.size(); ++i) {
      |         

Nearly all the time and log output is attributed to compiling cc_binary @protobuf//:protoc

[stagnation]

Great! We saw the expected improvements in the deployments repo, and it helped with some Windows issues we had at the time. I then forgot to add it to the other repos: buildbarn/bb-deployments#139

[EdSchouten]

Thanks!