T8390: Support manual workflow dispatch for package rebuild workflow

[kumvijaya]

Change Summary

Enable manual workflow triggering via workflow_dispatch in addition to automatic PR merge triggers. Updated the job condition to run when either a pull request is merged OR the workflow is manually dispatched, providing more flexibility for package rebuilds.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Other (please describe):

Related Task(s)

https://vyos.dev/T8390

Related PR(s)

Proposed changes

How to test

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[c-po]

This looks AI generated/assisted. I wo der why the headline states remove, but its oviously an extension / new conditional path.

[sever-sever]

Allow manual start of the workflow

[kumvijaya]

This looks AI generated/assisted. I wo der why the headline states remove, but its oviously an extension / new conditional path.

@c-po my bad, inititally it was remove only (yes, took ai assistance to do that writeup),.
then updated as per the discussion in slack, updated the PR info now