go: Introduce environment variable GO_NO_VENDOR_CHECKS to relax go vendor checks

This is used in https://github.com/tweag/gomod2nix to reconstruct a
vendor metadata file.

With the vendor checks we need a lot more metadata which isn't
relevant for building packages, especially since we've already locked
the dependency graph ahead of time

This has been ported from FreeBSD: https://reviews.freebsd.org/D24122

pkgs/development/compilers/go/1.15.nix

@@ -152,6 +152,7 @@ stdenv.mkDerivation rec {
     ./skip-external-network-tests-1.15.patch
     ./skip-nohup-tests.patch
     ./skip-cgo-tests-1.15.patch
+    ./go_no_vendor_checks.patch
   ] ++ [
     # breaks under load: https://github.com/golang/go/issues/25628
     (if stdenv.isAarch32