Fixed the default darwin xcrun toolchain logic.

Author: Rostepher

utils/build-script

@@ -18,6 +18,7 @@ import platform
 import sys
 import time
 
+from build_swift import defaults
 from build_swift import driver_arguments
 
 from swift_build_support.swift_build_support import (
@@ -1041,10 +1042,12 @@ def main_normal():
     shell.dry_run = args.dry_run
 
     # Prepare and validate toolchain
-    env_toolchain = os.environ.get('TOOLCHAINS')
     if args.darwin_xcrun_toolchain is None:
-        diagnostics.note('using toolchain {}'.format(env_toolchain))
-        args.darwin_xcrun_toolchain = env_toolchain
+        xcrun_toolchain = os.environ.get('TOOLCHAINS',
+                                         defaults.DARWIN_XCRUN_TOOLCHAIN)
+
+        diagnostics.note('Using toolchain {}'.format(xcrun_toolchain))
+        args.darwin_xcrun_toolchain = xcrun_toolchain
 
     toolchain = host_toolchain(xcrun_toolchain=args.darwin_xcrun_toolchain)
     os.environ['TOOLCHAINS'] = args.darwin_xcrun_toolchain