ci: make stale check manually triggerable (#688)

Signed-off-by: Patrick Stephens <pat@fluent.do>
Signed-off-by: Kilian Ries <mail@kilian-ries.de>

Author: patrick-stephens

.github/workflows/cron-stale.yaml

@@ -2,6 +2,7 @@ name: Close stale issues and PR(s)
 on:
   schedule:
     - cron: '30 1 * * *'
+  workflow_dispatch:
 
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