robots.txt: disallow access to snapshots

My dmesg is filled with the oom killer bringing down processes while the
Bingbot downloads every snapshot for every commit of the Linux kernel in
tar.xz format. Sure, I should be running with memory limits, and now I'm
using cgroups, but a more general solution is to prevent crawlers from
wasting resources like that in the first place.

Suggested-by: Natanael Copa <ncopa@alpinelinux.org>
Suggested-by: Julius Plenz <plenz@cis.fu-berlin.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>