Add public/ directory to CI cache

_cache/ includes information about which files in public/ are already
built, so if it is cached, then public/ should be cached too.

.gitlab-ci.yml

@@ -16,6 +16,7 @@ build:
     paths:
       - _cache
       - .stack
+      - public
   script:
     - stack exec site build
   except:
@@ -26,6 +27,7 @@ pages:
     paths:
       - _cache
       - .stack
+      - public
   script:
     - stack exec site build
   artifacts: