util/xmlconfig: add an env-var for overriding drirc search dir

This adds an environment variable that can be used to override the
global drirc serach directory. This can be useful for debugging, and
meson devenv, as used in the following commit.

Reviewed-by: Eric Engestrom <eric@igalia.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21808>

docs/envvars.rst

@@ -404,6 +404,12 @@ Core Mesa environment variables
 
       ``lspci -nn | grep VGA`` can be used to know the PCIe bus or ids to use.
 
+.. envvar:: DRIRC_CONFIGDIR
+
+   If set, overrides the global search-directories used when searching for
+   drirc config files. The user-local one will still be used. Mostly useful for
+   internal debugging.
+
 NIR passes environment variables
 --------------------------------