Alternative location for %appdata%\Nextcloud content

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### Feature description
Having Windows roaming profiles with folder redirection configured, the current config path location might lead to conflicts, especially for the containing cache folder and .db files.

Example for a common Windows domain environment with redirected user folders:
- `%appdata%`, which is usually `c:\users\myuser\appdata\roaming`, might point to a UNC path like `\\myshare\myuser\appdata`
- `%localappdata%` might point to its original location, e.g. `c:\users\myuser\appdata\local`

So if a user is logged into 2 Windows machines at the same time, 2 Nextcloud instances would access `\\myshare\myuser\appdata\nextcloud`.
This is e.g. great for the usage of `nextcloud.cfg` file, but not for logs, cache, *.db and possibly other instance-based data.

Some months ago, I tried it anyway. It resulted in lots of file conflicts and also data loss. (So I stayed with client version 2.x for a while, where I think the whole config folder resided in `%localappdata%`.)

In my opinion, `FOLDERID_RoamingAppData` should be only used (if at all) for `nextcloud.cfg[.backup_xx]`. Any other content of this directory should be placed under `%localappdata%` or `%temp%` etc.

As an alternative, it would be great to be able to set a custom config folder location via environment variables, e.g. `"NEXTCLOUDCFG=c:\test"`.

It would be great if you could consider this in a future version.

(As a workaround for the moment, I am using `portapps/nextcloud-portable`, deployed on a share. It uses the _share itself_ for the config store, and `%localappdata%\Nextcloud` for caching.)

Thank you!