Use dpkg-pixz multi-threaded xz decompressor

[ValdikSS]

This is a patch to use multi-threaded xz decompression of pixz in dpkg. In general, it saves some seconds when installing big packages (and it saves up to 4 seconds for 85 MB vscode .deb file on my laptop), but it's unfortunately rather useless for free Github workers since they run with 2 cores only.

I spent too much time to just drop these changes, but, well, it has only negligible performance improvement for the most Github users and it is cumbersome to store the binary data in the repo. It will probably save tens of seconds when installing large files or large amount of packages on a custom fat worker machines.

[ValdikSS]

It is also useless for upcoming Ubuntu 22.04 which has zstandard-compressed packages with multi-threading decompression.

[ValdikSS]

11 seconds vs 7 seconds in this run https://github.com/abbbi/github-actions-tune/actions/runs/1683412139

[ValdikSS]

So what do you think? Does the burden of supplying custom dpkg is not worth it?

[abbbi]

hi,

i think its a nice idea, but yes, shipping a custom dpkg if future ubuntu versions will handle this anyways is quite overhead..