fix build

Author: lunny

.drone.yml

@@ -64,6 +64,8 @@ steps:
   - name: build-without-gcc
     pull: always
     image: golang:1.11 # this step is kept as the lowest version of golang that we support
+    environment:
+      GO111MODULE: on
     commands:
       - go build -mod=vendor -o gitea_no_gcc # test if build succeeds without the sqlite tag