investigate powershell installer un-variabling PATH entries

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)

Start with the PowerShell installer code that reads and writes the registry-backed PATH value, then reproduce the CI case involving %USERPROFILE%/.cargo/bin. Done means existing PATH placeholders remain unexpanded and appropriate new entries use %USERPROFILE% where applicable, with CI validation covering the behavior.

Written by the indexing model from the issue text.