profiles: Gracefully deal with packages containing an etc/ symlink.

profiles: Gracefully deal with packages containing an etc/ symlink.

This fixes a bug whereby 'guix package -i gcc-toolchain' would fail in
'build-profile'.  This is because in 'gcc-toolchain', etc/ is a symlink,
and so the 'scandir' call in 'unsymlink' would return #f instead of
returning a list.

Reported by Andreas Enge <andreas.enge@inria.fr>.

* guix/build/profiles.scm (ensure-writable-directory)[unsymlink]: Append
  "/" to TARGET before calling 'scandir'.
* tests/profiles.scm ("etc/profile when etc/ is a symlink"): New test.