(current-filename) behaves differently in environment|build

[wldhx]

Hi guix,

Discovered following behaviour while setting up GNUnet CI:

Given

./gnu/packages/a.scm:
   ...
   (source (local-file (dirname (current-filename))))
   ...

export GUIX_PACKAGE_PATH=$PWD

`(current-filename)` returns `#f` when evaluated via `guix environment`,
while `guix build` does fine. Is this expected? Am I doing something wrong?

The usecase is like
https://lists.gnu.org/archive/html/help-guix/2017-04/msg00097.html.