Vendor protobuf in static builds.

[Ferroin]

Summary

There are issues detecting a system copy of Protobuf properly on newer versions of Alpine, and we’re functionally bundling whatever copy we link to for static builds anyway, so just use a vendored copy instead of a system copy.

This is a prerequisite for updating the base image used for building static builds to Alpine 3.20.

Test Plan

CI passes for this PR.