uninstalled: update XDG_DATA_DIRS to point to our devhelp index

When using hotdoc, one can build the documentation for a single subproject (eg. the GL plugins). In that case, hotdoc will look up links in devhelp indexes available in standard locations.

To make sure this case works, we thus need to add the path to our devhelp index to XDG_DATA_DIRS. This also means when running devhelp from inside the environment, the devhelp books produced by hotdoc will now show up.