postgresql.tests.postgresql.postgresql-backup-all: fix random dump

The new `\restrict` migitation creates random keys in the dump file by
default, which breaks a before/after test for the backup module. By
making the restrict key reproducible, the test passes again.

nixos/tests/postgresql/postgresql.nix

@@ -65,6 +65,7 @@ let
           services.postgresqlBackup = {
             enable = true;
             databases = lib.optional (!backupAll) "postgres";
+            pgdumpOptions = "--restrict-key=ABCDEFGHIJKLMNOPQRSTUVWXYZ";
           };
         };