swift: source darwin-sdk-setup.bash

Fixes builds using xcbuild that redefine SDKROOT in the xcbuild wrapper.
SDKROOT will be set using the same logic that other wrappers use.

(cherry picked from commit c578c48)

pkgs/development/compilers/swift/wrapper/wrapper.sh

@@ -12,6 +12,8 @@ cc_wrapper="${NIX_CC:-@default_cc_wrapper@}"
 
 source $cc_wrapper/nix-support/utils.bash
 
+source $cc_wrapper/nix-support/darwin-sdk-setup.bash
+
 expandResponseParams "$@"
 
 # Check if we should wrap this Swift invocation at all, and how. Specifically,