Use main repo for newest Ubuntu if Disco

Uses the main repo instead of updates as the updates repo
for the newest version may not have been populated yet.

Fixes https://github.com/antonym/netboot.xyz/issues/327

src/ubuntu.ipxe

@@ -32,6 +32,7 @@ goto deb_boot_type
 :mirrorcfg
 set mirrorcfg mirror/suite=${ubuntu_version}
 set dir ${ubuntu_base_dir}/dists/${ubuntu_version}-updates/main/installer-${arch_a}/current/images/netboot/
+iseq ${ubuntu_version} disco && set dir ${ubuntu_base_dir}/dists/${ubuntu_version}/main/installer-${arch_a}/current/images/netboot ||
 
 :deb_boot_type
 menu ${os} [${ubuntu_version}] Installer