libreoffice: wrapper: add hunspell dirs to DICPATH

LibreOffice doesn't know where to look for hunspell dictionaries on
NixOS but we can use the DICPATH env var to point to them. Now
spellchecking with system dictionaries works.

Note that some of these dictionaries are built from LibreOffice repos -
this is preferable to building them as part of LO as other applications
can use them.