run workflow subtemplates with new ScanContext

[tovask]

Proposed changes

When executing a workflow, the same ScanContext was used for all the Executers, and since the subtemplates in a workflow run in parallel, the OnResult callback for one template could be overwritten with another template's callback.
Running the subtemplates with a new ScanContext (keeping the same input, i.e.: targets, cookies, args) solves this problem.

fix #4933

Checklist

• Pull request is created against the dev branch
• All checks passed (lint, unit/integration/regression tests etc.) with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[GeorginaReeder]

Thanks so much for your contribution @tovask , we appreciate it!

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[Mzack9999]

lgtm - The shared contextargs.Context preserves the internal value-sharing functionality altough in different scan.Context

[tarunKoyalwar]

lgtm !