darwin.bootstrap_cmds: migrate to use darwin.sourceRelease

pkgs/os-specific/darwin/by-name/bo/bootstrap_cmds/package.nix

@@ -1,19 +1,27 @@
 {
   lib,
-  apple-sdk,
   bison,
   buildPackages,
   flex,
   meson,
   mkAppleDerivation,
+  pkgs,
   replaceVars,
   stdenv,
   stdenvNoCC,
 }:
 
 let
-  Libc = apple-sdk.sourceRelease "Libc";
-  xnu = apple-sdk.sourceRelease "xnu";
+  f =
+    pkgs: prev:
+    if !pkgs.stdenv.hostPlatform.isDarwin || pkgs.stdenv.name == "bootstrap-stage0-stdenv-darwin" then
+      prev.darwin.sourceRelease
+    else
+      f pkgs.stdenv.__bootPackages pkgs;
+  bootstrapSourceRelease = f pkgs pkgs;
+  # TODO(reckenrode): Use `sourceRelease` after migration has been merged and all releases updated to the same version.
+  Libc = bootstrapSourceRelease "Libc";
+  xnu = bootstrapSourceRelease "xnu";
 
   privateHeaders = stdenvNoCC.mkDerivation {
     name = "adv_cmds-deps-private-headers";