Change start script to point to nixpkgs-23.11-darwin

nixos-23.11 is at end of life, and until we switch to 24.05 or something
newer, we need to use the branch nixpkgs-23.11-darwin instead of
nixos-23.11. CI uses not only the nixpkgs specified in
nixpkgs/default.nix, it also uses the nixpkgs specified by the start
script. When attempting to use this nixpkgs on mac os, the CI runners
were failing due to a build error with python3.11-pytest-xdist-3.3.1.
Since 23.11 is EOL, there won't be any new commits to this branch fixing
the build issue with MacOS.

Author: iteratee

nixpkgs/default.nix

@@ -1,8 +1,8 @@
 { ... }@args:
 let
   # 2023-11-30
-  sha256 = "sha256:06m9r85brk6pcghm63ksl2zsx9j1kbagic3fx2p656z20v1bd48z";
-  rev = "781e2a9797ecf0f146e81425c822dca69fe4a348";
+  sha256 = "sha256:1lm7rkcbr7gg5zp62bga8iqyhg5hsvcly95hq0p3mcv7zq8n3wc2";
+  rev = "7144d6241f02d171d25fba3edeaf15e0f2592105";
 in
 import (fetchTarball {
   inherit sha256;

start

@@ -328,7 +328,9 @@ EOF
 		# https://github.com/tweag/rules_nixpkgs/blob/master/README.md
 		nixpkgs_git_repository(
 		    name = "nixpkgs",
-		    revision = "nixos-23.11",
+		    # The darwin channel tends to have cached packages for linux,
+		    # but not always vice versa.
+		    revision = "nixpkgs-23.11-darwin",
 		)
 
 		nixpkgs_cc_configure(