Avoid chrome_internal USE flag for arm64-generic.

Add hack to avoid adding the USE flag chrome_internal in the case of the
arm64-generic build target as it fails with this flag.

TEST=./run_tests
BUG=chromium:959931

Change-Id: Ia481a8abd7dc0ce1f030e02e937062ca47907b4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1597430
Reviewed-by: Evan Hernandez <evanhernandez@chromium.org>
Commit-Queue: David Burger <dburger@chromium.org>
Tested-by: David Burger <dburger@chromium.org>
diff --git a/scripts/build_api.py b/scripts/build_api.py
index c9bc120..12ebf8b 100644
--- a/scripts/build_api.py
+++ b/scripts/build_api.py
@@ -312,8 +312,20 @@
 
       extra_env = {
           'FEATURES': 'separatedebug',
-          'USE': 'chrome_internal',
       }
+
+      # TODO(crbug.com/959931): arm64-generic blows up on InstallPackages if
+      #  given the USE flag chrome_internal. Avoid adding the flag for
+      #  arm64-generic. Hack!!! This should be handled by config.
+      build_target = None
+      for descriptor in input_msg.DESCRIPTOR.fields:
+        field = getattr(input_msg, descriptor.name)
+        if isinstance(field, sysroot_pb2.Sysroot):
+          build_target = field.build_target.name
+          break
+      if not build_target == 'arm64-generic':
+        extra_env['USE'] = 'chrome_internal'
+
       try:
         result = cros_build_lib.RunCommand(cmd, enter_chroot=True,
                                            chroot_args=chroot_args,