meson.build: 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`.

meson.build

@@ -169,7 +169,7 @@ wireplumber_uninstalled = custom_target('wp-uninstalled',
 
 devenv = environment({
   'WIREPLUMBER_MODULE_DIR': builddir / 'modules',
-  'PIPEWIRE_CONFIG_DIR': srcdir / 'src' / 'config',
+  'WIREPLUMBER_CONFIG_DIR': srcdir / 'src' / 'config',
   'WIREPLUMBER_DATA_DIR': srcdir / 'src',
 })