Build/Test Tools: Support manual runs for the test old branch workflow.

This is the only workflow that does not currently support manually running.

See #62221.

git-svn-id: https://develop.svn.wordpress.org/trunk@59520 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

.github/workflows/test-old-branches.yml

@@ -12,6 +12,7 @@ on:
   schedule:
     - cron: '0 0 1 * *'
     - cron: '0 0 15 * *'
+  workflow_dispatch:
 
 # Disable permissions for all available scopes by default.
 # Any needed permissions should be configured at the job level.