This request is being tracked already:
Bug 909 – Expand environment variables in Open Directory/Location Profiles dialogs for local paths
You can vote for it there.

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When selecting the local directory, the path can be entered or selected by a directory selector.
If you enter an environment variable like %HOMEPATH%, the directory cannot be resolved, and the application displays an error.

We request a feature to also support local or system environment variables in local path.