ci: limit release to master branch

Jenkinsfile

@@ -23,6 +23,7 @@ pipeline {
 
         stage('Update emissary release') {
             when {
+                branch 'master'
                 expression {
                     return params.emissaryRelease != 'latest'
                 }
@@ -60,7 +61,7 @@ pipeline {
             }
         }
 
-        stage('Build and release') {
+        stage('Build') {
             steps {
                 script {
                     withCredentials([
@@ -80,6 +81,9 @@ pipeline {
         }
 
         stage('Release') {
+            when {
+                branch 'master'
+            }
             steps {
                 withCredentials([
                         usernamePassword([