run: Select a suitable backend for nested

This simplifies e.g. getting a nested phosh/phoc session up. We
prefer wayland over x11 backend if both are available. For
the drm case we have the `phoc-dev` scripts anyway.

run.in

@@ -4,6 +4,10 @@ set -e
 ABS_BUILDDIR='@ABS_BUILDDIR@'
 ABS_SRCDIR='@ABS_SRCDIR@'
 
+[ -z "${DISPLAY}" ] || WLR_BACKENDS=x11
+[ -z "${WAYLAND_DISPLAY}" ] || WLR_BACKENDS=wayland
+export WLR_BACKENDS
+
 export GSETTINGS_SCHEMA_DIR="${ABS_BUILDDIR}/data"
 set -x
 exec "${ABS_BUILDDIR}/src/phoc" "$@"