nixos/postgresqlBackup: add --rsyncable to compression programs

The --rsyncable option changes the behavior of gzip/zstd so that the
resulting files can be incrementally backed up easily. Tools like Borg,
rsync and xdelta can make use their deduplication/diff mechanisms more
easily.

In my local testing, this resulted in a 2% size increase for backup
files.

Signed-off-by: Sefa Eyeoglu <contact@scrumplex.net>
(cherry picked from commit 40c3d65)

nixos/modules/services/backup/postgresql-backup.nix

@@ -17,8 +17,8 @@ let
 
       compressCmd = getAttr cfg.compression {
         "none" = "cat";
-        "gzip" = "${pkgs.gzip}/bin/gzip -c -${toString cfg.compressionLevel}";
-        "zstd" = "${pkgs.zstd}/bin/zstd -c -${toString cfg.compressionLevel}";
+        "gzip" = "${pkgs.gzip}/bin/gzip -c -${toString cfg.compressionLevel} --rsyncable";
+        "zstd" = "${pkgs.zstd}/bin/zstd -c -${toString cfg.compressionLevel} --rsyncable";
       };
 
       mkSqlPath = prefix: suffix: "${cfg.location}/${db}${prefix}.sql${suffix}";