What does `go1.16 env GOROOT` give you? Did you compile `../../go-patch-overlay` with the same `go1.16` binary? If not, that's probably the cause of this issue.
When using with [`gomobile`](https://github.com/golang/mobile), setting `GOTOOLCHAIN=local` fixed compilation issue which I only ever saw in our GitHub Actions "build" workflow. Unfortunately, I don't remember if it was the exact same error as OP saw.
https://github.com/celzero/firestack/blob/91b781b115955eff57b142bb419392f8bddf054b/Makefile#L57