Location portal does not work on NixOS when no agent are installed

### What Happened?

*(Originally posted in https://github.com/elementary/seeds/issues/100#issuecomment-1562310351)*

Since pantheon-agent-geoclue2 is [not maintained](https://github.com/elementary/seeds/pull/100) anymore I am trying to [follow the change in NixOS](https://github.com/NixOS/nixpkgs/pull/233779/files) but got stuck.

I am actually trying to do some testing with flatpak `com.belmoussaoui.ashpd.demo`, for some reasons with the change I can make location requests but no longer can get a `LocationUpdated` response, this is not the case when I have **either** `pantheon-agent-geoclue2` **or** `geoclue-demo-agent` installed (natively). Since [ubuntu's geoclue-2.0 package provides `geoclue-demo-agent`](https://packages.ubuntu.com/jammy/amd64/geoclue-2.0/filelist), I am not sure if I should enable that in NixOS also.

We have geoclue 2.7.0 and https://github.com/elementary/default-settings/pull/279, https://github.com/elementary/switchboard-plug-security-privacy/pull/148 are backported when testing.

### Steps to Reproduce

1. Have no agent installed, including the demo one.
2. `flatpak run com.belmoussaoui.ashpd.demo`
3. Go to "location" and click "start session"
4. Receive no response.

### Expected Behavior

Receive `LocationUpdated` on `org.freedesktop.portal.Location`

### OS Version

Other Linux

### Software Version

Latest release (I have run all updates)

### Log Output

_No response_

### Hardware Info

_No response_