root: fix branch Name extraction to work on non PRs

scripts/az_do_set_branch.py

@@ -0,0 +1,11 @@
+"""Helper script to get the actual branch name, docker safe"""
+import os
+
+env_pr_branch = "SYSTEM_PULLREQUEST_SOURCEBRANCH"
+default_branch = "BUILD_SOURCEBRANCHNAME"
+
+branch_name = os.environ[default_branch]
+if env_pr_branch in os.environ:
+    branch_name = os.environ[env_pr_branch].replace("/", "-")
+
+print("##vso[task.setvariable variable=branchName]%s" % branch_name)