ci: pin the protoc plugins instead of installing @latest

these generate the committed stubs, so @latest makes codegen depend on
whatever released most recently. pinned to the versions proto/ records:
protoc-gen-go v1.36.11, protoc-gen-go-grpc v1.6.0.

.github/workflows/ci.yml

@@ -86,10 +86,14 @@ jobs:
           setup_only: true
           github_token: ${{ secrets.GITHUB_TOKEN }}
 
+      # Pinned, not @latest: these two write the committed stubs, so a floating
+      # version is an unreviewed input to generated code. The versions are the
+      # ones the stubs under proto/ record generating them, so what CI builds
+      # with and what is checked in stay the same thing.
       - name: Install protoc plugins
         run: |
-          go install google.golang.org/protobuf/cmd/protoc-gen-go@latest
-          go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@latest
+          go install google.golang.org/protobuf/cmd/protoc-gen-go@v1.36.11
+          go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@v1.6.0
           echo "$(go env GOPATH)/bin" >> "$GITHUB_PATH"
 
       - name: Cache Gradle