bug#23666: guix download fails for large files

[Andreas Enge]

Hello,

since I am regularly rebuilding texlive (who is not!), I would like to keep
the source tarball on my machine, and whenever I garbage collect it from the
store, add it back via
   guix download file:texlive-20150523-texmf.tar.xz

On my Novena with a 32 bit processor and 4 GB of main memory, this fails with
   guix download: error: sendfile: Broken pipe

The file itself has 1,8 GB.

Downloading the file via http as part of "guix build" works, and
"guix download file:" also works on my x86_64 machine with 8 GB of RAM.

Andreas