build: T3664: fix regression and bug in build script clone of vyos-1x repo

[jestabro]

This fixes a regression and a bug introduced in PR #585: the regression was dropping the keyword argument search_parent_directories=True in the initialization of the main (vyos-build) git repo; the bug was that re-running the build script with artifacts of a previous build would throw an error from the git clone of vyos-1x.

Change Summary

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):

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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