nix: don't use /tmp for the tests on darwin

Unlike on linux these are not namespaced per user so this will cause
build failures if /tmp/nix-test was not removed by a previous build if
the nixbld user id doesn't match by accident. Nix already creates a
unique tempdir for builds so we can use that instead.

Fixes #44172

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

@@ -71,7 +71,9 @@ let
     doInstallCheck = true; # not cross
 
     # socket path becomes too long otherwise
-    preInstallCheck = lib.optional stdenv.isDarwin "export TMPDIR=/tmp";
+    preInstallCheck = lib.optional stdenv.isDarwin ''
+      export TMPDIR=$NIX_BUILD_TOP
+    '';
 
     separateDebugInfo = stdenv.isLinux;