[WIP] fix(windows): invalid path when resolving killswitch directory

[rigor789]

PR Checklist

• [ ] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• [ ] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• [ ] You have signed the CLA.
• [ ] All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• [ ] Tests for the changes are included.

What is the current behavior?

What is the new behavior?

If an invalid process.env.USER (or SUDO_USER or USERNAME) is set, the CLI crashes.

Fixes/Implements/Closes #[Issue Number].