Prefer just-built compiler when building back-deployed concurrency library

Author: DougGregor

utils/swift_build_support/swift_build_support/products/backdeployconcurrency.py

@@ -128,7 +128,7 @@ def build(self, host_target):
         self.cmake_options.define(
             'SWIFT_DARWIN_DEPLOYMENT_VERSION_WATCHOS:STRING', '6.0')
 
-        self.build_with_cmake(["back-deployment"], build_variant, [])
+        self.build_with_cmake(["back-deployment"], build_variant, [], prefer_just_built_toolchain=True)
 
     def should_test(self, host_target):
         return False