Support ALSA_UCM_CONF environment variable to override UCM config.
PulseAudio (or Alsa?) has an environment variable ALSA_UCM_CONF to override the location of UCM configuration, e.g. https://github.com/NixOS/mobile-nixos/blob/45709499c6a4255b79739e01c258ce8f34fd5935/devices/pine64-pinephone/sound.nix#L17
It'd be great if Pipewire supported this.