build: fix xgo build after go1.14 go.mod update

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

.github/workflows/build.yml

@@ -227,7 +227,7 @@ jobs:
       - name: Cross-compile rclone
         run: |
           docker pull billziss/xgo-cgofuse
-          go get -v github.com/karalabe/xgo
+          GO111MODULE=off go get -v github.com/karalabe/xgo # don't add to go.mod
           xgo \
               -image=billziss/xgo-cgofuse \
               -targets=darwin/386,darwin/amd64,linux/386,linux/amd64,windows/386,windows/amd64 \