ci(bstein-home): run frontend test stage under bash

Jenkinsfile

@@ -262,8 +262,7 @@ PY
     stage('Frontend tests') {
       steps {
         container('frontend') {
-          sh '''
-            #!/usr/bin/env bash
+          sh(script: '''#!/usr/bin/env bash
 set -euo pipefail
 mkdir -p build
 cd frontend
@@ -272,7 +271,7 @@ PY
 npm run test:unit
 npm run test:component
 npm run test:e2e
-          '''
+''')
         }
       }
     }