Skip to content set `WIREPLUMBER_CONFIG_DIR` in devenv

This is needed for wireplumber to start up correctly in the meson
devenv when built as a subproject of pipewire. The reason for this
is that the value set for `PIPEWIRE_CONFIG_DIR` in the wireplumber
meson build file will be overriden when the pipewire meson files
set up another devenv with its own `PIPEWIRE_CONFIG_DIR`.

An alternative solution is to make pipewire support multiple directories in PIPEWIRE_CONFIG_DIR and use env.prepend() when setting PIPEWIRE_CONFIG_DIR in the pipewire devenv.

I am not sure which one is better. Is WIREPLUMBER_CONFIG_DIR deprecated? Is its use discouraged?

And I've just noticed that only sets WIREPLUMBER_CONFIG_DIR.

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