fix: add platform-specific executable extension to build output paths

[andreabedini]

• Import exeExtension from Distribution.Simple.BuildPaths to determine
correct executable naming by platform
• Extract platform information from toolchain configuration during build
orchestration
• Apply platform-specific file extension (e.g., .exe on Windows) when
constructing executable paths in build output
• Refactor variable naming for clarity (exe → exeName) to distinguish
filename base from full executable path
• Normalize import formatting for consistency

[hasufell]

src\Distribution\Client\ProjectOrchestration.hs:551:34: error: [GHC-88464]
    Variable not in scope:
      exeExtension :: Distribution.System.Platform -> String
    |
551 |         , let exe = exeName <.>  exeExtension platform
    |                                  ^^^^^^^^^^^^

[hasufell]

I made it compile with:

import Distribution.Simple.BuildPaths (exeExtension)