Fix error when building with Xcode 10

Xcode 10 deprecates the ARCH and CURRENT_ARCH env vars for
Run Script phases in favor of the ARCHS env var.

PiperOrigin-RevId: 199303769
diff --git a/src/TulsiGenerator/Scripts/bazel_build.py b/src/TulsiGenerator/Scripts/bazel_build.py
index db47da6..1b11836 100755
--- a/src/TulsiGenerator/Scripts/bazel_build.py
+++ b/src/TulsiGenerator/Scripts/bazel_build.py
@@ -536,9 +536,9 @@
 
     # Target architecture.  Must be defined for correct setting of
     # the --cpu flag
-    self.arch = os.environ.get('CURRENT_ARCH')
+    self.arch = os.environ.get('ARCHS')
     if not self.arch:
-      _PrintXcodeError('Tulsi requires env variable CURRENT_ARCH to be '
+      _PrintXcodeError('Tulsi requires env variable ARCHS to be '
                        'set.  Please file a bug against Tulsi.')
       sys.exit(1)