Revert "nixVersions.stable: 2.12 -> 2.13"

This reverts commit 956bc25a803ac00394f2a6d98b7a901ab3065dcf.

broke the tarball job

pkgs/tools/package-management/nix/default.nix

···
     sha256 = "sha256-uXh4+xjJUHQSCg+LHh6+SSYtMdjKQiTXMZ4uZFwzdq4=";
   };
 
-  stable = self.nix_2_13;
+  stable = self.nix_2_12;
 
   unstable = self.stable;
 })