configs/releng/airootfs/etc/mkinitcpio.conf.d/archiso.conf: use xz -9e

Use higher compression for the initramfs.

Now that mkinitcpio moves the compressed kernel modules and firmware files to the early uncompressed initramfs, we can compress the main initramfs image slightly more without it increasing the built time too much. This will increase the memory required for decompression from 9 MiB to 65 MiB (assuming the kernel's decompressor works the same as the xz tool), but that should not be an issue for any system targeting Arch.

This reduces the initramfs size by ~250 KiB, which will save ~500 KiB for the whole ISO.