archive: remove pgzip, use gzip from go std lib (MR 25)

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

inspired by: https://gitlab.com/postmarketOS/pmaports/-/issues/1704

internal/archive/archive.go

@@ -5,7 +5,7 @@ package archive
 
 import (
 	"bytes"
-	"compress/flate"
+	"compress/gzip"
 	"fmt"
 	"io"
 	"log"
@@ -17,7 +17,6 @@ import (
 	"syscall"
 
 	"github.com/cavaliercoder/go-cpio"
-	"github.com/klauspost/pgzip"
 	"gitlab.com/postmarketOS/postmarketos-mkinitfs/internal/filelist"
 	"gitlab.com/postmarketOS/postmarketos-mkinitfs/internal/osutil"
 )
@@ -222,10 +221,7 @@ func (archive *Archive) writeCompressed(path string, mode os.FileMode) error {
 		return err
 	}
 
-	gz, err := pgzip.NewWriterLevel(fd, flate.BestSpeed)
-	if err != nil {
-		return err
-	}
+	gz := gzip.NewWriter(fd)
 
 	if _, err = io.Copy(gz, archive.buf); err != nil {
 		return err