PS: Mark certain environment variables as not user controlled

[MathiasVP]

@chanel-y made an observation that I feel like I've only seen while reviewing some PowerShell query results: Certain environment variables are used very commonly, and they likely shouldn't be considered user input.

In 3a66283 you can see which environment variables I decided on excluding. @chanel-y I'd love to hear your thoughts on that list. In particular:

• Should we simply exclude any environment variable? Or should we only exclude the "known good ones" as I'm doing here?
• Are there any environment variables in the list that you feel like we shouldn't exclude?
• Are there any environment variables in the list that we should exclude?