plugin not found in $PATH

buf 1.14.0 is not finding plugins added to the PATH variable:

```
> PATH="$(pwd)/tools:${PATH}" buf generate
Failure: plugin go-grpc: exec: "": executable file not found in $PATH

> ls tools
buf			protoc-gen-go		protoc-gen-go-grpc
```

We use this approach to pin plugin versions. Downgrading to to 1.13.1 is our workaround for now.