ukify: switch from zstd to zstandard

ukify: switch from zstd to zstandard

The zstd library does not support stream decompression, and it
requires the zstd header to contain extra metadata, that the kernel
build does not append:

$ file -k vmlinuz-6.13+unreleased-cloud-arm64
vmlinuz-6.13+unreleased-cloud-arm64: PE32+ executable (EFI application) Aarch64 (stripped to external PDB), for MS Windows, 2 sections\012- data
$ ukify build --linux vmlinuz-6.13+unreleased-cloud-arm64 --initrd /boot/initrd.img-6.12.12-amd64 --output uki
Kernel version not specified, starting autodetection 😖.
Real-Mode Kernel Header magic not found
+ readelf --notes vmlinuz-6.13+unreleased-cloud-arm64
readelf: Error: Not an ELF file - it has the wrong magic bytes at the start
Traceback (most recent call last):
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 2508, in <module>
    main()
    ~~~~^^
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 2497, in main
    make_uki(opts)
    ~~~~~~~~^^^^^^
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 1326, in make_uki
    opts.uname = Uname.scrape(linux, opts=opts)
                 ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 382, in scrape
    version = func(filename, opts=opts)
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 372, in scrape_generic
    text = maybe_decompress(filename)
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 219, in maybe_decompress
    return get_zboot_kernel(f)
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 199, in get_zboot_kernel
    return cast(bytes, zstd.uncompress(f.read(size)))
                       ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
zstd.Error: Input data invalid or missing content size in frame header.

This appears to be by design:

https://github.com/sergey-dryabzhinsky/python-zstd/issues/53

Switch to python3-zstandard, which works.