xbps-src: introduce XBPS_REPO_COMPTYPE for etc/conf.

This sets the repository data compression format, as explained
in xbps-rindex(1). By default set to `gzip'.

etc/defaults.conf

@@ -83,6 +83,12 @@ XBPS_SUCMD="sudo /bin/sh -c"
 #
 #XBPS_PKG_COMPTYPE=none
 
+# [OPTIONAL]
+# Set the repository compression format (defaults to gzip/zlib). See xbps-rindex(1)
+# for the available formats.
+#
+#XBPS_REPO_COMPTYPE=zstd
+
 # [OPTIONAL]
 # Enable or disable global package build options, these options apply
 # to all packages that support the matching options.