Modified conditions for workflow kick-off

Previous conditions for workflow kick-off [push, pull-request] were
causing start of two simultaneous workflows when new commit was pushed
to an already open pull request. After the change such situation should
start only one workflow.

.github/workflows/client.yml

@@ -1,7 +1,11 @@
 name: Go
 
 #TODO: extend the conditions once workflow gets tested together with other workflows 
-on: [push, pull_request]
+on:  
+  push:
+    branches:
+      - master
+  pull_request:
 
 jobs:
   build-and-test: