test: (ArchLinux) Replace initramfs-linux.img with initramfs-linux-fallback.img.

test: (ArchLinux) Replace initramfs-linux.img with initramfs-linux-fallback.img.

Currently /boot/initramfs-linux.img is used as the default initrd for ArchLinux.
Although, since the kernel modules that are not necessary for the host  environment are removed from
initramfs-linux.img by mkinitcpio 's autodetect hook, the kernel modules necessary for qemu may be missing.
(ata_piix, ext4, and so on in my case.)
As a result, the test environment may not be built properly and the test will be failed.

initramfs-linux-fallback.img will skip this autodetect hook, so the test will run successfully in more
environments.

Both initramfs-linux.img and initramfs-linux-fallback.img are generated by default.