releaser: use -mod=vendor, not go mod download

.goreleaser.yml

@@ -2,13 +2,12 @@
 # Make sure to check the documentation at http://goreleaser.com
 before:
   hooks:
-    # You may remove this if you don't use go modules.
-    - go mod download
-    # you may remove this if you don't need go generate
     - go generate ./...
 builds:
   - env:
       - CGO_ENABLED=0
+    flags:
+      - -mod=vendor
     goos:
       - linux
       - windows