Use parallel gzip instead of gzip for compression

golangs compress/gzip isn't a parallel implementation, so it's quite a
bit slower on most modern servers then pgzip. The below benchmark
run shows that publishing a debian bullseye mirror snapshot (amd64, arm64,
armhf, source) shows a gain of about 35% in publishing time (when skipping
bz2 using MR #1081)

```
 hyperfine -w 1 -m 3 -L aptly aptly-nobz2,aptly-nobz2-pgzip -p "{aptly} -config aptly.conf publish drop bullseye || true" "{aptly} -config aptly.conf  publish snapshot --skip-bz2=true --skip-contents --skip-signing bullseye"
Benchmark 1: aptly-nobz2 -config aptly.conf  publish snapshot --skip-bz2=true --skip-contents --skip-signing bullseye
  Time (mean ± σ):     35.548 s ±  0.378 s    [User: 39.465 s, System: 10.046 s]
  Range (min … max):   35.149 s … 35.902 s    3 runs

Benchmark 2: aptly-nobz2-pgzip -config aptly.conf  publish snapshot --skip-bz2=true --skip-contents --skip-signing bullseye
  Time (mean ± σ):     26.592 s ±  0.069 s    [User: 42.207 s, System: 9.676 s]
  Range (min … max):   26.521 s … 26.660 s    3 runs

Summary
  'aptly-nobz2-pgzip -config aptly.conf  publish snapshot --skip-bz2=true --skip-contents --skip-signing bullseye' ran
    1.34 ± 0.01 times faster than 'aptly-nobz2 -config aptly.conf  publish snapshot --skip-bz2=true --skip-contents --skip-signing bullseye'
```

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Upstream MR: https://github.com/aptly-dev/aptly/pull/1082