Make GOPATH shared between build steps

.drone.yml

@@ -2,6 +2,13 @@ kind: pipeline
 name: default
 
 steps:
+  - name: test_draftbot
+    image: golang
+    commands:
+      - cd ./draftbot
+      - GOPROXY=https://modules.fromouter.space go mod download
+      - CGO_ENABLED=0 go test .
+
   - name: build_draftbot
     image: golang
     commands:
@@ -28,4 +35,8 @@ steps:
       event:
         - push
         - tag
-      branch: master
+      branch: master
+
+volumes:
+  - name: gopath
+    temp: {}