Previously, before replying to a /nar/* request, 'guix publish' would first
build up the whole nar into memory (as a consequence of
<http://bugs.gnu.org/21093>), which obviously doesn't scale.
* guix/scripts/publish.scm (render-nar): Return STORE-PATH instead of a
procedure that calls 'write-file'.
(sans-content-length): New procedure.
(http-write): For 'x-nix-archive', don't call '%http-write'. Instead, call
'write-file' right from here, using BODY as the file name.
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