Gitlab CI: work around cassava problem, use system-ghc

.gitlab-ci.yml

 image: haskell:8
+before_script:
+  - stack upgrade
 build:
-  script: "stack build"
+  script:
+    - /root/.local/bin/stack build

stack.yaml

@@ -21,6 +21,8 @@ extra-deps:
       - gogol
       - gogol-apps-calendar
 
+system-ghc: true
+
 # Nix support
 nix:
   enable: true