build: T6923: use the Debian mirror for security updates if it's specified explicitly

[dmbaturin]

Change Summary

If the user runs sudo ./build-vyos-image --debian-mirror $URL, use $URL for all Debian mirror URLs in lb config.

It's still possible to specify different mirrors for "Debian mirror" (--debian-mirror), "Debian security mirror" (--debian-security-mirror), and mirror for pbuilder (--pbuilder-debian-mirror) if needed.

But in most cases if someone specifies a custom mirror, they usually want it for all roles and trying to make them specify everything explicitly that live-build allows to specify explicitly is redundant.

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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• 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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