investigate powershell installer un-variabling PATH entries

[Gankra]

In the powershell installer we load, edit, and save the registry value storing (part of) the user's PATH. In testing in CI I noticed that the value of $env:UserProfile (one of the windows equivs of $HOME) in the current env was appearing in the result. This suggest to me that the values in ci were like %USERPROFILE%/.cargo/bin and in the process of reading/editing we forced those to get substituted permanently, degrading their quality.

We should:

• not be substituting existing values
• ideally be writing %USERPROFILE% values ourselves when appropriate (as the shell installer does)