Fix the regex bug for env variables

[sudo-arash]

What's the problem this PR addresses?

The bug is that, If your ENV variable needs to have a value with ${} in it, yarn will error:

Usage Error: Environment variable not found 

on windows.

Related issue: #6491

How did you fix it?

I simply updated the regex and the way the variable variableValue is validated.

Checklist

• [x] I have read the Contributing Guide.

• [x] I have set the packages that need to be released for my changes to be effective.

• [x] I will check that all automated PR checks pass before the PR gets reviewed.

[arcanis]

Thanks! Can you add a test?

[sudo-arash]

Hi

Thanks for the comment! I sadly don't know Type Script nor JavaScript tests. But I do know python ones(Unit tests).

Sincerely, Arash