[CI] Set MIX_ENV to test in `variables` section instead of specifying it before the command. Fix `mix compile` compiling in the dev env.

.gitlab-ci.yml

@@ -8,6 +8,7 @@ variables:
   POSTGRES_USER: postgres
   POSTGRES_PASSWORD: postgres
   DB_HOST: postgres
+  MIX_ENV: test
 
 cache:
   key: ${CI_COMMIT_REF_SLUG}
@@ -23,15 +24,15 @@ before_script:
   - mix local.rebar --force
   - mix deps.get
   - mix compile --force
-  - MIX_ENV=test mix ecto.create
-  - MIX_ENV=test mix ecto.migrate
+  - mix ecto.create
+  - mix ecto.migrate
 
 lint:
   stage: lint
   script:
-    - MIX_ENV=test mix format --check-formatted
+    - mix format --check-formatted
 
 unit-testing:
   stage: test
   script:
-    - MIX_ENV=test mix test --trace
+    - mix test --trace