swift-corelibs: set NIX_COREFOUNDATION_RPATH in a hook

Closes #230870. Thanks to @eliasnaur for the test case and for rasining
awareness and to @veprbl for the work done on #111385.

This takes a slightly different approach from those two PRs. The hook is
set unconditionally. The stdenv bootstrap doesn’t really need CF at all,
so setting the hook is harmless. This simplifies things.

pkgs/os-specific/darwin/swift-corelibs/corefoundation.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchFromGitHub, fetchurl, ninja, python3, curl, libxml2, objc4, ICU }:
+{ lib, stdenv, fetchFromGitHub, fetchurl, makeSetupHook, ninja, python3, curl, libxml2, objc4, ICU }:
 
 let
   # 10.12 adds a new sysdir.h that our version of CF in the main derivation depends on, but
@@ -104,4 +104,6 @@ stdenv.mkDerivation {
       ln -s Versions/Current/$i $base/$i
     done
   '';
+
+    darwinEnvHook = makeSetupHook { name = "darwin-env-hook"; } ./pure-corefoundation-hook.sh;
 }

pkgs/os-specific/darwin/swift-corelibs/pure-corefoundation-hook.sh

@@ -0,0 +1,7 @@
+usePureCoreFoundation() {
+# Avoid overriding value set by the impure CF
+    if [ -z "${NIX_COREFOUNDATION_RPATH:-}" ]; then
+        export NIX_COREFOUNDATION_RPATH=@out@/Library/Frameworks
+    fi
+}
+addEnvHooks "$hostOffset" usePureCoreFoundation