bug#22536: Download progress display

[Leo Famulari]

As reported on #guix by user ilit, when the terminal is very narrow, the
download progress line is constantly reprinted, as in this excerpt:

$ guix build -S libreoffice
substitute: warning: failed to install locale: Invalid argument
The following file will be downloaded:
   /gnu/store/yv821gn5z48g19mzjcvb4dfcxljjsg8q-libreoffice-5.0.5.2.tar.xz
@ substituter-started 
/gnu/store/yv821gn5z48g19mzjcvb4dfcxljjsg8q-libreoffice-5.0.5.2.tar.xz 
/gnu/store/9p7mf2a21c6a5k06jgbb9yg9abx7ilh9-guix-0.10.0-0.8062/libexec/guix/substitute
warning: failed to install locale: Invalid argument

Found valid signature for 
/gnu/store/yv821gn5z48g19mzjcvb4dfcxljjsg8q-libreoffice-5.0.5.2.tar.xz
>From 
>https://4606.nsupdate.info/nar/yv821gn5z48g19mzjcvb4dfcxljjsg8q-libreoffice-5.0.5.2.tar.xz
Downloading yv821g...-libreoffice-5.0.5.2.tar.xz (159.4MiB installed)...
 libreoffice-5.0.5.2.tar.xz 2.7MiB/s 00:00 | 104KiB transferre 
libreoffice-5.0.5.2.tar.xz 2.7MiB/s 00:00 | 112KiB transferre 
libreoffice-5.0.5.2.tar.xz 2.9MiB/s 00:00 | 120KiB transferre 
libreoffice-5.0.5.2.tar.xz 2.7MiB/s 00:00 | 128KiB transferre 
libreoffice-5.0.5.2.tar.xz 2.8MiB/s 00:00 | 136KiB transferre 
libreoffice-5.0.5.2.tar.xz 2.8MiB/s 00:00 | 144KiB transferre 
libreoffice-5.0.5.2.tar.xz 2.9MiB/s 00:00 | 152KiB transferre 
libreoffice-5.0.5.2.tar.xz 3.0MiB/s 00:00 | 160KiB transferre lib