When writing a shebang script, you expect your path to be relative to
the script, not the cwd. We previously handled this correctly for
relative file paths, but not for expressions.

This handles both -p & -E args. My understanding is this should be
what we want in any cases I can think of - people run scripts from
many different working directories. @edolstra is there any reason to
handle -p args differently in this case?

Fixes NixOS#4232