Run linting on an image with golint preinstalled

Running `go get` adds a line to `go.mod` every time, so using a
preinstalled golint avoids churn.

.drone.yml

@@ -9,8 +9,10 @@ steps:
     commands:
       - go test ./cmd/... ./internal/...
       - go vet ./cmd/... ./internal/...
-      - go get -u golang.org/x/lint/golint
-      - golint ./cmd/... ./internal/...
+  - name: lint
+    image: cytopia/golint
+    commands:
+      - golint -set_exit_status ./cmd/... ./internal/...
   - name: build
     image: golang:1.13
     commands: