The xgo builds for macOS, Linux and Windows are used for testing - the
actual builds are built on the correct platform.
Since the darwin build has stopped working, this can be an excuse for
removing these builds as they really are only for testing.
The Android and IOS builds will continue to be built by xgo
See: https://github.com/billziss-gh/cgofuse/issues/47
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
