CI: move CI env variable to the root of the workflow. (#27)

This way we match the Travis CI behavior, and it's inherited by all steps.

Author: XhmikosR

.github/workflows/test.yml

@@ -1,5 +1,7 @@
 name: Tests
 on: [push, pull_request]
+env:
+  CI: true
 
 jobs:
   test:
@@ -22,8 +24,6 @@ jobs:
 
       - name: Install npm dependencies
         run: npm ci
-        env:
-          CI: true
 
       - name: Run tests
         run: npm run test-ci