buildGoModule: disable consult the checksum database on build

Since Go 1.13, `GOSUMDB` defaults to "sum.golang.org", to consult the
checksum database of the main module's go.sum.

We already use the default behavior when building `go-modules`, but Go
tries to consult the checksum database again when building the module,
and fails because since it requires `cacert` and `git` which are not
propagated when building the package.

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

@@ -98,6 +98,7 @@ let
 
       export GOCACHE=$TMPDIR/go-cache
       export GOPATH="$TMPDIR/go"
+      export GOSUMDB=off
       export GOPROXY=file://${go-modules}
 
       runHook postConfigure