fix(ci) Sets the DB_HOST environment variable for the MySQL test job to 'mysql'.

This allows the test suite to correctly connect to the MySQL service during CI.

Author: rrd108

.github/workflows/ci.yml

@@ -65,6 +65,8 @@ jobs:
 
       - name: Test
         run: npm run test:mysql
+        env:
+          DB_HOST: mysql
 
   test-postgresql:
     runs-on: ubuntu-latest