archive: remove pgzip, use gzip from go std lib

This replaces the parallel gzip with the boring gzip from Go's standard lib. The main motivations for doing this are:

1. Reduce runtime memory requirements

2. shed some external dependencies

There's obviously a trade-off with compression speed/time (as seen below), but I feel like it's a worthwhile trade-off.

Note that there's likely very little impact to boot performance wrt extracting these archives, the compression levels are similar.

Measured on a Shift 6mq, which is a very fast phone...

** compress/gzip:

User time (seconds): 1.81
System time (seconds): 0.38
Percent of CPU this job got: 104%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:02.09
Maximum resident set size (kbytes): 62024

-rw-r--r-- 1 clayton clayton 6.1M Sep 20 17:20 initramfs
-rw-r--r-- 1 clayton clayton 2.5M Sep 20 17:20 initramfs-extra

** pgzip:

User time (seconds): 1.19
System time (seconds): 0.48
Percent of CPU this job got: 159%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:01.05
Maximum resident set size (kbytes): 109952

-rw-r--r-- 1 clayton clayton 6.8M Sep 20 17:20 initramfs
-rw-r--r-- 1 clayton clayton 2.8M Sep 20 17:20 initramfs-extra