nix: revive nix_2_6

The critical regression bug in
NixOS/nix#6572 does not appear (at least as
often) in nix 2.6 according to the discussion there, so we need to
allow people to pin this older version.

Partial revert of
b2eea05

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

@@ -50,7 +50,13 @@ in lib.makeExtensible (self: {
 
   nix_2_5 = throw "nixVersions.nix_2_5 has been removed";
 
-  nix_2_6 = throw "nixVersions.nix_2_6 has been removed";
+  # This is the last version of nix where https://github.com/NixOS/nix/issues/6572
+  # is not an unsolved breaking bug. Please keep it until the issue is fixed
+  # and in latest stable nix.
+  nix_2_6 = common {
+    version = "2.6.1";
+    sha256 = "sha256-E9iQ7f+9Z6xFcUvvfksTEfn8LsDfzmwrcRBC//5B3V0=";
+  };
 
   nix_2_7 = common {
     version = "2.7.0";