chore: allow triggering blackduck scans manually (#1104)

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#### What this PR does / why we need it

it is currently impossible to trigger the blackduck scans manually so
now we should be able to in case we have a transient failure.

#### Which issue(s) this PR fixes
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Usage: `Fixes #<issue number>`, or `Fixes (paste link of issue)`.
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.github/workflows/blackduck_scan_scheduled.yaml

@@ -2,6 +2,7 @@ name: Blackduck Scan Cronjob
 on:
   schedule:
     - cron:  '5 0 * * 0'
+  workflow_dispatch:
 
 permissions:
   checks: write