buildGoModule: prefer verbose output when NIX_DEBUG set

> The -v flag causes vendor to print the names of vendored modules and packages
> to standard error.

The `-v` flag helped me debug a `buildGoModule` derivation yesterday evening.

pkgs/development/go-modules/generic/default.nix

@@ -106,7 +106,10 @@ let
       mkdir -p "''${GOPATH}/pkg/mod/cache/download"
       go mod download
     '' else ''
-      go mod vendor
+      if (( "''${NIX_DEBUG:-0}" >= 1 )); then
+        goModVendorFlags+=(-v)
+      fi
+      go mod vendor "''${goModVendorFlags[@]}"
     ''}
 
       mkdir -p vendor