Use better compression algorithm for pax files

The pax files containing the port releases are compressed using the compress algorithm (.Z). This is an ancient, inefficient compression algorithm that has long been supplanted by better choices. I suggest that we only use .Z for the minimal bootstrapping packages, and instead use .xz for everything else. This will significantly reduce both download time and space on disk for the package cache. It has the downside of making xz a dependency on the toolchain, but I think that's a reasonable tradeoff.

~ > ls -l dos2unix-master.20231114_165830.zos.pax.Z 
-rw-r-----   1 ANGIO    DBX       741888 Dec  6 08:18 dos2unix-master.20231114_165830.zos.pax.Z
~ > gunzip dos2unix-master.20231114_165830.zos.pax.Z 
~ > xz dos2unix-master.20231114_165830.zos.pax 
~ > ls -l dos2unix-master.20231114_165830.zos.pax.xz 
-rw-r-----   1 ANGIO    DBX       182528 Dec  6 08:18 dos2unix-master.20231114_165830.zos.pax.xz