Unable to boot Alpine Linux kernels >=5.15

It seems Alpine has switched the kernel to zstd compression, and our initramfs can't handle it.

### postmarketOS initramfs ###
Configuring kernel firmware image search path
Kernel does not support setting the firmware image search path. Skipping.
[    0.708835] Module has invalid ELF structures
[    0.710084] Module has invalid ELF structures
modprobe: can't load module virtio (kernel/drivers/virtio/virtio.ko.zst): invalid module format
[    0.713426] Module has invalid ELF structures
[    0.716154] Module has invalid ELF structures
modprobe: can't load module virtio (kernel/drivers/virtio/virtio.ko.zst): invalid module format
[    0.719575] Module has invalid ELF structures
[    0.720934] Module has invalid ELF structures
modprobe: can't load module firmware_class (kernel/drivers/base/firmware_loader/firmware_class.ko.zst): invalid module format
[    0.725460] Module has invalid ELF structures
[    0.726964] Module has invalid ELF structures
modprobe: can't load module virtio (kernel/drivers/virtio/virtio.ko.zst): invalid module format
[    0.730806] Module has invalid ELF structures
[    0.732238] Module has invalid ELF structures
modprobe: can't load module evdev (kernel/drivers/input/evdev.ko.zst): invalid module format
[    0.735580] Module has invalid ELF structures
[    0.736917] Module has invalid ELF structures
modprobe: can't load module jbd2 (kernel/fs/jbd2/jbd2.ko.zst): invalid module format
modprobe: module usb_f_rndis not found in modules.dep
Trying to mount subpartitions for 10 seconds...
[    0.769995] Module has invalid ELF structures

This is happening on edge with a clean chroot.

CC @craftyguy