ncurses: build with the bootstrap stdenv on Darwin

Using the bootstrap stdenv avoids an infinite recursion from xcbuild
depending on ncurses depending on xcrun from xcbuild, which is
propagated by the non-bootstrap stdenv.

pkgs/top-level/all-packages.nix

···
   ncurses =
     if stdenv.hostPlatform.useiOSPrebuilt
     then null
-    else callPackage ../development/libraries/ncurses { };
+    else callPackage ../development/libraries/ncurses {
+      # ncurses is included in the SDK. Avoid an infinite recursion by using a bootstrap stdenv.
+      stdenv =
+        if stdenv.isDarwin then
+          darwin.bootstrapStdenv
+        else
+          stdenv;
+    };
 
   ndi = callPackage ../development/libraries/ndi { };