component/firefox: set $GSETTINGS_SCHEMA_DIR in the wrapper

This fixes a crash happening after `setFile` selenium command under
some conditions (it does not always happen, but I did not investigate
in which conditions it happens exactly).

[Parent 94283, Main Thread] ###!!! ASSERTION: No GSettings schemas are installed on the system: 'glib assertion', file /builds/worker/checkouts/gecko/toolkit/xre/nsSigHandlers.cpp:164

component/firefox/buildout.cfg

@@ -32,6 +32,7 @@ part = firefox
 recipe = slapos.recipe.build
 location = ${buildout:bin-directory}/${:part}
 fonts-conf = ${firefox-default-fonts-conf:output}
+gsettings-schema-dir = ${gtk-3:location}/share/glib-2.0/schemas/
 init =
   self.buildout[options['part']]
 install =
@@ -45,12 +46,14 @@ install =
   # BBB use a default fonts.conf for compatibility, but it's software instance
   # responsability to build a fonts.conf with the fonts they want.
   [ "$FONTCONFIG_FILE" ] || export FONTCONFIG_FILE=%s
+  [ "$GSETTINGS_SCHEMA_DIR" ] || export GSETTINGS_SCHEMA_DIR=%s
   exec $d/firefox "$@"
   """ % (
     part['location'],
     ':'.join(part['library'].split()),
     ':'.join(part['path'].split()),
     options['fonts-conf'],
+    options['gsettings-schema-dir'],
   ))
     os.fchmod(f.fileno(), 0o755)