configs/releng/profiledef.sh: use zstd compression for bootstrap image

The zstd tool has now been around for a while, so the availability of
it should not be a concern anymore.

Unlike gzip which was used until now, zstd offers higher compression
while still being faster (and multi-threaded).
The `--auto-threads=logical` option is used just so that there is some
difference between the releng and baseline profiles.

Everyone using the official Arch Linux bootstrap tarball (previously
`archlinux-bootstrap-YYYY.MM.DD-x86_64.tar.gz` or
`archlinux-bootstrap-x86_64.tar.gz`) will need to update their scripts
and etc. to use `archlinux-bootstrap-YYYY.MM.DD-x86_64.tar.zst` or
`archlinux-bootstrap-x86_64.tar.zst` instead.

Implements https://gitlab.archlinux.org/archlinux/archiso/-/issues/130

CHANGELOG.rst

@@ -14,6 +14,8 @@ Added
 Changed
 -------
 
+- Change releng profile's bootstrap tarball compression from gzip to zstd. zstd provides higher and faster compression.
+
 Deprecated
 ----------
 

configs/releng/profiledef.sh

@@ -15,7 +15,7 @@ arch="x86_64"
 pacman_conf="pacman.conf"
 airootfs_image_type="squashfs"
 airootfs_image_tool_options=('-comp' 'xz' '-Xbcj' 'x86' '-b' '1M' '-Xdict-size' '1M')
-bootstrap_tarball_compression=(gzip -cn9)
+bootstrap_tarball_compression=('zstd' '-c' '-T0' '--auto-threads=logical' '--long' '-19')
 file_permissions=(
   ["/etc/shadow"]="0:0:400"
   ["/root"]="0:0:750"