app: revert to "gimp-paintbrush-tool" as default tool.

A UI unit test (paintbrush_is_standard_tool()) was expecting the
paintbrush tool to be the default tool. This actually does not fit with
my tests as the crop tool was always the default tool if I were to
delete my config folder and start anew. Yet it was working for the unit
testing at the very least since `make check` used to work until now, and
was broken by my change.

Anyway with my new code (commit 31e38fe869) and this additional one, now
it should work both for unit tests and real usage. So this commit fixes
both the CI and the originally expected default tool.

For the record, Aryeom also agrees that paintbrush is a good default
tool for first usage/discovery of GIMP.

app/widgets/gimpdeviceinfo.c

@@ -853,7 +853,7 @@ gimp_device_info_set_default_tool (GimpDeviceInfo *info)
   tools     = GIMP_TOOL_PRESET (info)->gimp->tool_info_list;
   tool_info =
     GIMP_TOOL_INFO (gimp_container_get_child_by_name (tools,
-                                                      "gimp-rect-select-tool"));
+                                                      "gimp-paintbrush-tool"));
 
   if (info->priv->device)
     {