gnu: gajim: Honour $GAJIM_PLUGIN_PATH.

gnu: gajim: Honour $GAJIM_PLUGIN_PATH.

This allows Gajim to find plug-ins installed with Guix.
Reported at <http://issues.guix.gnu.org/44730>.

However, at the time of pushing both packaged plug-ins (gajim-omemo and
gajim-openpgp) require an unstable version of Gajim and will not load.

* gnu/packages/messaging.scm (gajim)[arguments]: Replace the
‘add-plugin-dirs’ substitution phase with a...
[source]: ...patch that won't silently break in the future.
* gnu/packages/patches/gajim-honour-GAJIM_PLUGIN_PATH.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.