patch: let cmd/go be built statically and still make cgo binaries from that static binary

[bradfitz]

Summary: let us build tailscale/go's cmd/go as a static binary so it runs on NixOS, but then still be able to build cgo binaries with it. Upstream status: should but haven't. Importance: required for NixOS users inside the company, including some of our builders Example prior commit: a2f29de0a91a04521eaef084ab7eefd319d4fbbe

(Then used in 26292c0a2e9823c4216e8776912a43fd9e50d444, 178d6bce616ceefcdd687341cb74d7d8868877a7, etc)

cc @danderson