gnu: webkitgtk: Expose all font locations to the bubblewrap sandbox.

gnu: webkitgtk: Expose all font locations to the bubblewrap sandbox.

Fixes <https://issues.guix.gnu.org/41174> and <https://issues.guix.gnu.org/42618>.

The webkitgtk library relied on by Epiphany and others uses bubblewrap to
sandbox its process.  This sandbox was only exposing fonts relative to
XDG_DATA_HOME.  Extend it so that it honors font locations specified via
XDG_DATA_DIRS as well, which is what Guix uses.

* gnu/packages/patches/webkitgtk-bind-all-fonts.patch: Add patch.
* gnu/local.mk: Register it.
* gnu/packages/webkit.scm (webkitgtk)[source]<patches>: Use it.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>