feat(dracut): search for zstd compressor first

feat(dracut): search for zstd compressor first

Searching for an available compressor was introduced in commit
693b7a32833b ("Cleanup compressor handling") but the order was not
justified.

Ubuntu uses zstd for compressing the packages, the kernel modules,
firmware files, and the initrd (when using initramfs-tools). zstd has a
good compromise between compression ratio and speed. Therefore search
for zstd first.

Further readings:

 * https://lists.ubuntu.com/archives/ubuntu-devel/2021-December/041726.html
 * analysis.ipynb on https://github.com/waveform80/compression