fix(system): Update TargetEnvVar instead of PATH

[xworld21]

Description

The functions Add-Path, Remove-Path admit an argument -TargetEnvVar which would update the correct profile or system environment variable, but would always act on PATH in the current session.

I simply replaced $env:PATH with Get/Set-Content "env:$TargetEnvVar" to fix this. I have also updated the message 'Adding (...) to your path', so there will be a visible change to 'Adding (...) to your PATH'.

(I suppose this doesn't merit an entry in the CHANGELOG... does it?)

Motivation and Context

Closes #6579.

How Has This Been Tested?

I have checked the functions manually in the terminal, including with non-existing variables (hence the -ErrorAction Ignore).

Checklist:

• I have read the Contributing Guide.
• I have ensured that I am targeting the develop branch.
• I have updated the documentation accordingly.
• I have updated the tests accordingly.
• I have added an entry in the CHANGELOG.