Publish prereleases as such (#893)

From the docs (https://goreleaser.com/customization/release/#github):
If set to auto, will mark the release as not ready for production in
case there is an indicator for this in the tag e.g. v1.0.0-rc1.

.goreleaser.yml

@@ -44,5 +44,6 @@ release:
   github:
     owner: mattermost
     name: mattermost-load-test-ng
+  prerelease: auto
   name_template: "{{ .ProjectName }}-v{{ .Version }}"
   disable: false