sh-4.3$ ./pre-inst-env guix build --source libgit2 --substitute-urls=https://mirror.hydra.gnu.org
substitute: updating list of substitutes from 'https://mirror.hydra.gnu.org'... 100.0%
2.8 MB will be downloaded:
   /gnu/store/s62d5lbr6sb7x0mxhhdwf13in7yi8mbc-libgit2-0.26.0.tar.xz
substitute: updating list of substitutes from 'https://mirror.hydra.gnu.org'... 100.0%
@ substituter-started /gnu/store/s62d5lbr6sb7x0mxhhdwf13in7yi8mbc-libgit2-0.26.0.tar.xz /gnu/store/vir3lrwqy50pr8fkaf3m091dgbrja2n6-guix-0.13.0/libexec/guix/substitute
Downloading https://mirror.hydra.gnu.org/guix/nar/s62d5lbr6sb7x0mxhhdwf13in7yi8mbc-libgit2-0.26.0.tar.xz (2.7MiB installed)...
 libgit2-0.26.0.tar.xz  2.7MiB                                                                                                  2.3MiB/s 00:01 [####################] 100.0%

@ substituter-succeeded /gnu/store/s62d5lbr6sb7x0mxhhdwf13in7yi8mbc-libgit2-0.26.0.tar.xz
/gnu/store/s62d5lbr6sb7x0mxhhdwf13in7yi8mbc-libgit2-0.26.0.tar.xz