When setting FlutterProjectArgs.command_line_argv prior to launching the
engine, we were previously setting a placeholder value rather than the
executable name. This resulted in Platform.executable (from dart:io)
returning "placeholder" in application code.

This updates the Windows implementation for consistency with macOS and
guarantees that Platform.executable will return a reasonable value in
Dart code.

Note that this does not affect Platform.resolvedExecutable, which returns
a full, resolved path, and is implemented in the Dart runtime itself.
Previously the code:

print(Platform.executable);
print(Platform.resolvedExecutable);

resulted in the following output on Windows:

flutter: placeholder
flutter: C:\path\to\project\build\windows\runner\Debug\project.exe

after this patch, it results in:

flutter: project.exe
flutter: C:\path\to\project\build\windows\runner\Debug\project.exe

Issue: flutter/flutter#83921

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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