Before this change xgo was getting added to go.mod - the build then failed with
go: inconsistent vendoring in /usr/src/rclone:
github.com/karalabe/xgo@v0.0.0-20191115072854-c5ccff8648a7: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt
This change gets xgo in GOPATH mode to avoid it getting added to go.mod