Vendor protoc so the build needs no system protobuf compiler

The gRPC build script required a system protoc, which a fresh clone and
the CI container lack (build failed with 'Could not find protoc'). Set
PROTOC from protoc-bin-vendored in build.rs (unless one is already set),
so cargo build works out of the box everywhere.

build.rs

@@ -1,4 +1,10 @@
 fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // Use a vendored protoc, so `cargo build` needs no system protobuf compiler
+    // (a fresh clone, CI, and the deploy box all build out of the box). Respect
+    // an explicit PROTOC if one is already set.
+    if std::env::var_os("PROTOC").is_none() {
+        std::env::set_var("PROTOC", protoc_bin_vendored::protoc_bin_path()?);
+    }
     // Server only: the Rust side never needs to dial itself as a gRPC client.
     // Django (or any other consumer) generates its own stub from proto/echo.proto.
     tonic_build::configure()