file-based program document enhancement

[WeihanLi]

This pull request updates the documentation for running file-based C# apps on Unix systems and revises the recommended version for the System.CommandLine NuGet package. The changes clarify how to resolve the dotnet executable path and ensure security by suggesting the latest stable package version.

Improvements to Unix execution instructions:

• Updated the guidance for locating the dotnet executable: replaced the incorrect whence dotnet command with where dotnet, and added an alternative using the shebang #!/usr/bin/env dotnet for automatic PATH resolution.

Documentation update for package versioning:

• Changed the recommended version of System.CommandLine from 2.0.0-beta6 to the stable 2.0.0, ensuring users are directed to use the latest secure release.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/fundamentals/tutorials/file-based-programs.md	Tutorial: Build file-based C# programs

[Copilot]

Pull Request Overview

This PR updates the System.CommandLine package version reference and adds documentation for using the env command in shebang configuration for Unix-based systems.

• Updates System.CommandLine version from 2.0.0-beta6 to 2.0.0
• Adds documentation for using #!/usr/bin/env dotnet as an alternative shebang configuration
• Changes the Unix command reference from whence dotnet to where dotnet

[BillWagner]

Thanks @WeihanLi

I'll now.