Add a search path for ZATHURA_PLUGIN_PATH.
This patch allow users to look up the plugin directory by an environment variable.
This is useful for distributions such as NixOS and Guix, where each package installs to an isolated read-only directory, and profiles are composed by adding symlinks to the respective package outputs.
Thus, the plugin path is something like
~/.guix-profile/lib/zathura, but Zathura itself is installed to
/gnu/store/mwiyi6nkkm82i50fhb7jyixss1mlswcf-zathura-0.3.9 and plugins live in similar unpredictable locations.