[native_assets_builder] Fix env var typo

[dcharkes]

Thanks for the report @MichealReed!

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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Unrelated files missing license headers

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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 86.676%. remained the same
when pulling be42ea1 on user-profile
into 8132054 on main.