[native_assets_cli] Cleanup JSON: env_script and env_script_arguments

[dcharkes]

Bug:

• [native_assets_cli] Remove fallback JSON encodings #2199

Cleanup of:

• Remove CCompiler.{envScript,envScriptArgs} and replace by other means #1606

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
native_assets_cli	Breaking	0.16.0	0.17.0-wip	0.17.0	✔️

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ✔️

// 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.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

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

[goderbauer]

LGTM

[coveralls]

coverage: 77.298% (-0.2%) from 77.475%
when pulling 3d8d361 on json-cleanup-envscript
into f36662f on main.