If your dpkg-vendor configuration in base-files is an Ubuntu derivative,
but not exactly Ubuntu, and not an exact match for anything in vendor/,
we want to use vendor/ubuntu.
Previously we used whatever $DISTRO happened to be at the end of the
"for" loop above, but that's undesirable: even if we assume that find(1)
returns results in alphabetical order (which is not guaranteed), we
wouldn't want to return a non-Ubuntu result if a future apt version
happens to have a vendor that sorts later than vendor/ubuntu, for
example vendor/zombieos or something.
Signed-off-by: Simon McVittie <email@example.com>