libpkgx sets XDG_DATA_DIRS in some cases

[jhheider]

it appears that this interferes with the proper functioning of e.g. gsettings from GNOME. since tools use the standard-defined defaults if the value is unset, i think we need to consider that if we add values to it, we must first add the standards-defined fallback to prevent changing behavior. see the below thread for an example of how it interfere's with GNOME's tooling.

          Oh. Maybe we have a bug in pkgx then?

❯ env | grep XDG
XDG_RUNTIME_DIR=/run/user/1002/

❯ pkgx env | grep XDG
XDG_DATA_DIRS=/home/felipecrs/.pkgx/gnu.org/coreutils/v9.5.0/share
XDG_RUNTIME_DIR=/run/user/1002/

If pkgx was not supposed to alter XDG_ vars in runtime, then it was not supposed to inject XDG_DATA_DIRS either, I suppose.

Originally posted by @felipecrs in pkgxdev/pantry#5991 (comment)

[mxcl]

Sounds right. What's the default?

[jhheider]

If $XDG_DATA_DIRS is either not set or empty, a value equal to /usr/local/share/:/usr/share/ should be used.

ref: https://specifications.freedesktop.org/basedir-spec/latest/index.html#variables