GDM 50 fails with custom XDG_CONFIG_HOME set in pam_env.conf
For a long time, I've had the following snippet in `/etc/security/pam_env.conf`:
```
XDG_CONFIG_HOME DEFAULT=@{HOME}/local/config
```
However, after a recent update that included upgrading to GNOME 50, GDM now no longer presents a login greeter and instead automatically loads the GNOME shell as the `gdm-greeter` user.
I've narrowed it down to this setting in `pam_env.conf`. I can only start the GNOME shell normally with `XDG_CONFIG_HOME` unset, or set to `@{HOME}/.config` (the default value). No other value works.
`XDG_DATA_HOME` and other XDG Base Directory variables can be set without issue, and after login they show up in `env` like normal.
I'm running Arch Linux with GDM version `50.0-2`.
issue
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