Use EV prefix for evince environment setup

See https://gitlab.gnome.org/GNOME/evince/-/commit/395312868baf4ee03c5118ee5c58f25601c2af1d

bin/tryton

@@ -14,7 +14,7 @@ if hasattr(sys, 'frozen'):
     prefix = os.path.dirname(sys.executable)
     os.environ['GTK_EXE_PREFIX'] = prefix
     os.environ['GTK_DATA_PREFIX'] = prefix
-    os.environ['EVINCE_BACKENDS_DIR'] = prefix
+    os.environ['EV_BACKENDS_DIR'] = prefix
     os.environ['XDG_DATA_DIRS'] = os.path.join(prefix, 'share')
     etc = os.path.join(prefix, 'etc')
     os.environ['GDK_PIXBUF_MODULE_FILE'] = os.path.join(