postinst: Fix check for Ubuntu

Martin Pittrequested to merge
mpitt/libvirt-dbus:fix-ubuntu-check into debian/latest

dpkg-vendor lives in dpkg-dev, it cannot be used in postinst scripts. Check os-release for "ubuntu" instead, which will match "ID=ubuntu" in Ubuntu itself, and "ID_LIKE=ubuntu" in derivatives such as Mint.

Fixes: cd5b637d

LP: #1802005

Edited by Martin Pitt

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