Fix VS Code cargo executable probing

Handle Windows executable suffixes when the VS Code extension probes
for cargo through PATH or CARGO_HOME, and preserve case-insensitive
environment variable lookup for supplied env objects so copied Windows
`Path` entries still resolve cargo.

Normalize and document the Windows CARGO_HOME test comparison as well,
because VS Code path resolution can lowercase the drive letter in
`fsPath` without changing which executable was found.

This keeps the extension-side lookup aligned with the Rust-side
toolchain helper behavior and avoids breaking create-project on common
Windows setups.

AI-assisted: OpenAI Codex was used to identify and apply this change.