GitLab CI: forgot the `-y` on apt-get

.gitlab-ci.yml

@@ -3,8 +3,8 @@ image: haskell:8
 # https://github.com/commercialhaskell/stackage/issues/3132
 # https://github.com/snoyberg/http-client/issues/292
 before_script:
-  - apt-get update
-  - apt-get install build-essential
+  - apt-get -y update
+  - apt-get -y install build-essential
   - stack upgrade
 build:
   # For some reason, just `stack` uses the old version, so explicitly