CI: Install protobuf on FreeBSD

```
cargo build --quiet --example cln-plugin-startup
  error: failed to run custom build command for `cln-grpc v0.4.0 (/home/runner/work/lightning/lightning/cln-grpc)`

  Caused by:
    process didn't exit successfully: `/home/runner/work/lightning/lightning/target/debug/build/cln-grpc-1c0900b8d6f448d4/build-script-build` (exit status: 101)
    --- stdout
    cargo:rerun-if-changed=proto/node.proto
    cargo:rerun-if-changed=proto

    --- stderr

    thread 'main' panicked at cln-grpc/build.rs:7:10:
    called `Result::unwrap()` on an `Err` value: Custom { kind: NotFound, error: "Could not find `protoc`. If `protoc` is installed, try setting the `PROTOC` environment variable to the path of the `protoc` binary. Try installing `protobuf-compiler` or `protobuf` using your package manager. It is also available at https://github.com/protocolbuffers/protobuf/releases  For more information: https://docs.rs/prost-build/#sourcing-protoc" }
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
  gmake: *** [cln-rpc/Makefile:15: target/debug/examples/cln-plugin-startup] Error 101
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

.github/workflows/bsd.yml

@@ -43,6 +43,7 @@ jobs:
             lowdown \
             pkgconf \
             jq \
+            protobuf \
             curl
 
             python3.10 -m ensurepip