drone.yml: even more gomobile commands to make it work

.drone.yml

@@ -59,6 +59,12 @@ steps:
       branch: trunk
       event: [ push, pull_request ]
     commands:
+      - rm -rf /gomogile/pkg*
+      - rm -rf /go/pkg
+      - rm -rf /tmp/gomobile*
+      - rm -rf vendor
+      - go clean --modcache
+      - go mod download
       - go get golang.org/x/mobile/cmd/gomobile
       - go get golang.org/x/mobile/cmd/gobind
       - go get -u golang.org/x/mobile