ci: build protoc into the crate instead of installing it per job (#10830)

Every workflow that builds the Rust volume server first installed protoc
from a package manager - twelve steps across apt, brew and choco. That is
37s per job on a good day, and this week archive.ubuntu.com stalled long
enough for four jobs to burn their whole timeout without reaching a build.

protoc-bin-vendored ships the compiler as a build-dependency, so it now
arrives through the cargo registry the workflows already cache and there
is nothing left to install. cargo build works on a machine with no protoc
at all, which is worth as much locally as it is in CI.

It also pins the version. The apt protoc on ubuntu-22.04 is 3.12, old
enough to reject proto3 optional, which is why build.rs passes
--experimental_allow_proto3_optional; the vendored one is 31.1. The flag
stays, since it costs nothing and keeps a build against an older PROTOC
working, and an explicit PROTOC still overrides the vendored binary for
packagers who supply their own.

.github/workflows/container_dev.yml

@@ -31,9 +31,6 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v7
 
-      - name: Install protobuf compiler
-        run: sudo apt-get update && sudo apt-get install -y protobuf-compiler
-
       - name: Install Rust toolchain
         uses: dtolnay/rust-toolchain@stable
         with: