Fine-tuning GitHub Actions

[YanWenKun]

1. Use LZMA2 algorithm for 7zip compression:

LZMA2 is a variant of LZMA, it uses significantly more RAM, and ~70% faster (trade space for time).

My test result:

 -m0=lzma2 -mx=5 -mfb=32 -md=16m -ms=on -mf=BCJ2
 8m 53s 
 1.39 GB package

 -m0=lzma2 -mx=7 -mfb=64 -md=32m -ms=on -mf=BCJ2
 10m 30s 
 1.36 GB package

 -m0=lzma2 -mx=8 -mfb=64 -md=32m -ms=on -mf=BCJ2
 10m 48s 
 1.36 GB package

I didn't test -mx=9 -mfb=64 -md=64m, the gain would be marginal.

RAM usage on GitHub Actions looks totally fine.

I've opted for -mx=8 in the commit.

2. Updating GitHub Actions versions

Underneath is simply a node.js runtime version bump from 16 to 20. Tested and confirmed no issues to be concerned about.

[YanWenKun]

BTW I made a big package and tried some different approach on packaging. But I guess it would be cumbersome and disruptive to contribute to upstream. So just FYI