CI: replace apt-get protoc with cached action

[iainmcgin]

Five CI jobs run sudo apt-get install -y protobuf-compiler, ~14s each.

Four of them (check, test, clippy, doc) genuinely need protoc:
they build --workspace, which includes tests/streaming, which has a
connectrpc-build build.rs. The fifth (examples) runs
./examples/multiservice/test.sh, which only builds
-p multiservice-example - checked-in generated code, no protoc
required.

The conformance job had the same unnecessary install removed in #5.

Proposed fix

• Drop the apt-get from examples - pure dead weight.
• For the four jobs that need protoc, switch from apt-get to
  arduino/setup-protoc@v3. It downloads the release zip and caches
  across runs - first hit ~2s, cache hits near-instant. apt-get pulls
  a pile of transitive deb packages every time.

~14s saved per job on the critical path. Not huge, but it's the same
14s four times for every PR.