os-prober generates incorrect entries for Arch linux

[Carlos Pita]

You can see a detailed description of this issue in https://askubuntu.com/questions/628206/how-can-i-boot-into-arch-linux-using-initramfs-in-ubuntus-grub

The fix suggested there by Naiky is simple and works with a little modification described in comment https://askubuntu.com/questions/628206/how-can-i-boot-into-arch-linux-using-initramfs-in-ubuntus-grub#comment2176155_1144517

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: os-prober 1.74ubuntu2
ProcVersionSignature: Ubuntu 5.4.0-48.52-generic 5.4.60
Uname: Linux 5.4.0-48-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.9
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Mon Oct 12 18:24:24 2020
InstallationDate: Installed on 2020-04-24 (171 days ago)
InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
SourcePackage: os-prober
UpgradeStatus: No upgrade log present (probably fresh install)