ci: make pahole-staging workflow manually triggerable

Allow to manually trigger pahole-staging workflow.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

.github/workflows/pahole.yml

@@ -1,10 +1,10 @@
 name: pahole-staging
 
 on:
+  workflow_dispatch:
   schedule:
     - cron:  '0 18 * * *'
 
-
 jobs:
   vmtest:
     runs-on: ubuntu-20.04