protoc tests do not run on Windows

Currently protoc tests don't pass on Windows.

Trying to run the bzlmod test results in this error:

$ bazelisk test //...

Executing tests from //:root_proto_lint
-----------------------------------------------------------------------------
'..' is not recognized as an internal or external command,
operable program or batch file.

Getting this working likely requires a bit of finagling as Bazel has poor support for formulating exec calls on its own and typically relies on sh a lot. Since our dependencies already require an MSys2 installation that Bazel is aware of (via BAZEL_SH) we should also feel free to rely on this to simplify our lives (though personally, I prefer to lean on PowerShell if we can easily do so.)