Speed up cargo eclass SRC_URI generation
From the PMS:
> mirror
> The package’s SRC_URI entries may not be mirrored, and
> mirrors should not be checked when fetching.
The crate dependencies may very well be mirrored, judging by their
licenses. To fetch distfiles from their original locations immediately,
set `GENTOO_MIRRORS=`.
https://devmanual.gentoo.org/general-concepts/manifest/#generating-the-manifest
Signed-off-by: Lucio Sauer <watermanpaint@posteo.net>