Update gitlab ci configuration

Expecting the changes to do the builds faster.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>

.gitlab-ci.yml

@@ -1,27 +1,16 @@
-image: haskell:latest
+image: haskell:8.6.3
 
-before_script:
-  - apt-get update && apt-get install xz-utils make
-  - export STACK_ROOT=`pwd`/.stack
-  - stack setup
-  - stack install --only-dependencies
-  - stack build
-
-build:
-  cache:
-    paths:
-      - _cache
-      - .stack
-  script:
-    - stack exec site build
-  except:
-    - master
+variables:
+  STACK_ROOT: $CI_PROJECT_DIR/.stack
 
 pages:
   cache:
     paths:
       - _cache
       - .stack
+  before_script:
+    - stack install --only-dependencies
+    - stack build
   script:
     - stack exec site build
   artifacts: