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.

CHANGELOG.rst

@@ -18,6 +18,9 @@ Changed
   when using FAT32 (needs less than 1 MiB) and to give more space for adding custom files when repacking an ISO (e.g.
   when preparing it for Secure Boot).
 - Remove 300 KiB padding needed for CDs if the ISO exceeds the maximum size of a CD.
+- Use ``xz -9e`` as the releng profile's initramfs compression. Now that mkinitcpio does not decompress the loadable
+  kernel modules and firmware files anymore and moves them to the early uncompressed initramfs, we can compress the main
+  initramfs image with a higher compression without it having much impact on the ISO build time.
 
 Deprecated
 ----------

configs/releng/airootfs/etc/mkinitcpio.conf.d/archiso.conf

@@ -1,2 +1,3 @@
 HOOKS=(base udev microcode modconf kms memdisk archiso archiso_loop_mnt archiso_pxe_common archiso_pxe_nbd archiso_pxe_http archiso_pxe_nfs block filesystems keyboard)
 COMPRESSION="xz"
+COMPRESSION_OPTIONS=(-9e)