flowey: fix more issues with running vmm-tests under wsl

[chris-oo]

Two additional fixes for running cross compiled windows under WSL for vmm-tests. This now allows running something like cargo xflowey vmm-tests --target windows-x64 --dir /mnt/q/vmm_tests_out end to end, and having it build and execute all tests within WSL.

[Copilot]

Pull request overview

This PR fixes issues with running cross-compiled Windows VMM tests under WSL. It enables end-to-end execution of cargo xflowey vmm-tests --target windows-x64 --dir /mnt/q/vmm_tests_out within WSL by ensuring proper environment variable propagation and path validation.

Changes:

• Added WSLENV configuration to propagate environment variables when running Windows executables from WSL
• Added validation to ensure Windows-accessible paths are used when targeting Windows from WSL
• Made running_in_wsl() function public in flowey_cli for reuse across crates

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
flowey/flowey_lib_hvlite/src/run_prep_steps.rs	Adds WSLENV handling to propagate environment variables when running Windows prep_steps executable from WSL
flowey/flowey_hvlite/src/pipelines/vmm_tests.rs	Validates output directory is Windows-accessible when targeting Windows from WSL
flowey/flowey_cli/src/lib.rs	Makes running_in_wsl() function public and adds documentation