Fix running plugin host on electron windows

[Tom-Yucw]

What it does

Closes #6846

to run plugins created by yo @theia/plugin on windows.

How to test

1. Using yo @theia/plugin to make a Hello World plug-in
2. Adding nodejs version like "@types/node": "12.20.0" in package.json to avoid error "node_modules@types\node\assert.d.ts"
3. Run yarn to build plugin
4. Run "Hosted Plug-in: Start Instance" in command, and select the source directory which includes file package.json

A new instance is showed up successfully on windows.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

The changes look good to me. I can confirm that the issue exists on master and is addressed by the changes nicely 👍

[msujew]

@Tom-Yucw Please don't create a new PR for every change, and simply add your changes to this PR. If you choose to amend to your first commit, you can force push them using git push -f.

[vince-fugnitto]

@msujew should we merge for this week's release?