So the store itself is still located in '/', it's in '/gnu/store'. However, the location can be changed when building guix (you would need to compile it yourself) with the ./configure options --with-store-dir={PATH}. (The /var/guix path you referred to is the state path, where guix stores accounting databases and stuff for it to actually track the store)

EDIT: I forgot to mention, do not do this. If you want the store in a different location, set up a bind mount. Guix makes assumptions that would make it so you no longer could use substitutes (binary downloads of packages), so keep that in mind.[1]

[1]: https://lists.gnu.org/archive/html/help-guix/2022-02/msg0016...