Better use of notification address

Adresses: https://www.jenkins.io/doc/book/pipeline/jenkinsfile/#string-interpolation

.ci/Jenkinsfile

@@ -43,8 +43,8 @@ pipeline {
         }
         always {
             withCredentials([string(credentialsId: 'jed-notification-email', variable: 'EMAIL')]) {
-                mail to: "${EMAIL}",
-                     subject: "[${currentBuild.fullDisplayName}] Build #${currentBuild.number}: ${currentBuild.currentResult}",
+                mail to: '$EMAIL',
+                     subject: "[${currentBuild.fullDisplayName}] ${currentBuild.currentResult}",
                      body: "Duration: ${currentBuild.durationString} / Jenkins URL: ${env.BUILD_URL}"
             }
         }