* guix/scripts/perform-download.scm (perform-download): Add
#:print-build-trace? and pass it to 'url-fetch'.
(guix-perform-download): Define 'print-build-trace?' and pass it to
'perform-download'.
* guix/build/download.scm (ftp-fetch): Add #:print-build-trace? and
honor it.
(url-fetch): Likewise.
* nix/libstore/builtins.cc (builtinDownload): Set _NIX_OPTIONS
environment variable.
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