XDG_CONFIG_HOME ignored

### Problem

Running `beet` with `XDG_CONFIG_HOME` set:

```sh
$ export $XDG_CONFIG_HOME=~/etc
$ beet
```

results in a config directory being created under `~/.config`, instead of the correct location, `~/etc`.
If there is a text file named `~/.config/beets`, `beet` crashes with the following error message:

```
error: database file /home/username/.config/beets/library.db cannot not be opened: unable to open database file
```

(Also, `library.db` should probably be in a different base directory, but that is a separate issue).

### Setup

* OS: OpenBSD 7.6 and NixOS 24.05 (but should be reproducible on any Unix-like system)
* Python version: 3.11.9
* beets version: 1.6.0
* Turning off plugins made problem go away (yes/no): no
* Configuration: none/default (but shouldn't matter)

This may actually be a bug in [confuse](https://github.com/beetbox/confuse) rather than in beets. Beet seems to work correctly with a patched confuse.