xbps-src: support multiple pkg compression formats via etc/conf.

XBPS_PKG_COMPTYPE can be set via etc/conf to set your preferred compression
format; defaults to xz, like xbps-create(1).

The "none" setting is only available since 0.48 (unreleased).

etc/defaults.conf

@@ -73,6 +73,15 @@ XBPS_SUCMD="sudo /bin/sh -c"
 #
 #XBPS_DEBUG_PKGS=yes
 
+# [OPTIONAL]
+# Set the package compression format. Available formats:
+#  - gzip
+#  - bzip2
+#  - xz (default)
+#  - none (available since xbps-0.48)
+#
+#XBPS_PKG_COMPTYPE=none
+
 # [OPTIONAL]
 # Enable or disable global package build options, these options apply
 # to all packages that support the matching options.