mock: disable networking in bootstrap for hermetic builds #1697

Timeline protection: Good to go

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Please, refer to the following document for more details on how to
craft a great change note for inclusion with your pull request:
https://rpm-software-management.github.io/mock/Release-Notes-New-Entry

The following news fragments found: [<PatchedFile: releng/release-notes-next/dont-fail-for-non-existing-resolv-conf.bugfix.md>]

Pattern: re.compile('releng/release-notes-next/(?P<issue_number>[^\\./]+)\.(?P<fragment_type>breaking|feature|bugfix|config)(\.\d+)?(\.[^\\./]+)*$')