Fix variable in daily slack notifications (#6221)

Currently the report fails with variables[Build.SourceBranchName]:
command not found which is obviously not what we want (it’s mixing up
the syntax in Azure’s yaml config and Bash). Looking at the
code in the tell-slack-failed.yml, this one does seem to work but I
haven’t tested this so :crossed-fingers:.

changelog_begin
changelog_end

ci/daily_tell_slack.yml

@@ -16,7 +16,7 @@ steps:
         MESSAGE=${{ parameters['success-message'] }}
     fi
     PAYLOAD="{\"text\":\"$MESSAGE\n\"}"
-    if [ "$(variables['Build.SourceBranchName'])" = "master" ]; then
+    if [ "$(Build.SourceBranchName)" = "master" ]; then
         curl -XPOST \
              -i \
              -H 'Content-type: application/json' \