build: T6903: allow passing vyos-1x repo path in an environment variable

[dmbaturin]

Change Summary

Make it possible to pass a custom vyos-1x git repository URL via VYOS1X_REPO_URL environment variable, for cases when image build requires vyos1-x Python libs from a fork rather than the mainline repo.

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)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Component(s) name

Build scripts.

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

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