vendor/getinfo: Don't assume that Ubuntu is the last vendor

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.

Closes: #924662
Signed-off-by: Simon McVittie <smcv@collabora.com>

vendor/getinfo

@@ -28,7 +28,7 @@ getcurrent() {
 
 	# Do the ubuntu/debian dance we talked about
 	if dpkg-vendor --derives-from ubuntu; then
-		echo $DISTRO
+		echo ubuntu
 		return 0
 	fi