Replace pixz with pigz for faster RAUC bundle creation?

[yfyf]

Currently RAUC bundles are compressed using pixz. This takes very long, on my machine:

real    12m9.685s
user    36m54.183s
sys     0m18.055s

The speed-up with pigz is 10x:

real    1m17.513s
user    3m30.048s
sys     0m6.234s

This eliminates the indexing support that pixz provides, but is it used anywhere?

To ensure that nothing breaks, it's best to do this after #200 is merged - this way we can verify that older releases properly handle the differently-compressed RAUC bundles.

We can also be even more adventurous and replace xz (i.e. LZMA) with zstd for broader improvements in space efficiency as well.