ci: Enable caching of stack artifacts

This should reduce the build time in CI drastically.

.gitlab-ci.yml

 image: haskell:8.2
 
+# Change stacks global directory to be inside the project directory,
+# since we can only cache local dependencies.
+variables:
+  STACK_ROOT: "$CI_PROJECT_DIR/.stack-root"
+
+cache:
+  paths:
+    - .stack-root/
+
 test:
   script: stack --no-terminal --system-ghc --no-install-ghc build --test --haddock --no-haddock-deps