[native_toolchain_c] Support environment for cl.exe already set up

[dcharkes]

We'd like flutter_tools to invoke the hook/build.dart with the environment for cl.exe already set up and stop passing in input.config.code.cCompiler.envScript.

Therefore, package:native_toolchain_c should not go search for an vcvars batch script if the compiler has been provided in the code config but a vcvars script has not been provided.

Issue:

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

[github-actions]

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Breaking changes ✔️

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Changes to files need to be accounted for in their respective changelogs.

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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.

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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

[coveralls]

coverage: 87.728% (+0.01%) from 87.717%
when pulling 03e1851 on toolchain-env-vars
into a646680 on main.