app-antivirus/clamav-unofficial-sigs: new revision with systemd timer.

app-antivirus/clamav-unofficial-sigs: new revision with systemd timer.

For the same reason we introduced an /etc/cron.d job in the previous
revision, it makes sense to include a systemd timer. The optimal
settings for the update script are a bit tricky to get right, and
since we run as the "clamav" user, they're both non-default and not
entirely under our control.

I haven't tested this (no systemd), but what could possibly go wrong?

Bug: https://bugs.gentoo.org/694120
Package-Manager: Portage-2.3.76, Repoman-2.3.16
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>