fix: preserve PATHEXT for Windows cached tasks

[SegaraRai]

Fixes #365.

Preserves PATHEXT in the default Windows environment passthrough so PowerShell package shims can resolve node.exe during cached task execution.

Why PATHEXT is required

In the issue repro, marker-cli is exposed through a pnpm-generated node_modules/.bin/marker-cli.ps1 shim. On Windows, that shim sets $exe = ".exe" and then falls back to invoking Node by name:

& "node$exe" "$basedir/../marker-cli/bin/marker-cli.mjs" $args

So the actual command is & "node.exe" ....

When PATHEXT is missing from the sanitized cached-task environment, PowerShell can still locate node.exe with Get-Command node.exe, but native invocation by name does not run correctly: & "node.exe" --version produces no output, $LASTEXITCODE is unset, and the repro marker file is not created. Restoring PATHEXT with .EXE makes the same shim execute normally and create the marker.

This is why preserving PATHEXT fixes the cached PowerShell shim path without changing fspy or the shim rewrite itself.

Verified with the issue repro on Windows, cargo fmt --check, cargo build -p vite_task_bin, and cargo test -p vite_task_plan --test plan_snapshots -- windows_cmd_shim_rewrite.

[branchseer]

Thanks!