syncriscv: add role for mirroring the RISC-V port

Going to be served by all our Geo boxes under riscv.mirror.pkgbuild.com.

group_vars/all/geo.yml

 geo_acme_dns_challenge_ns: redirect.archlinux.org
 geo_domains:
   - geo.mirror.pkgbuild.com
+  - riscv.mirror.pkgbuild.com
 # geo_options.*.hosts defaults to "{{ groups['geo_mirrors'] }}"
 geo_options:
   geo.mirror.pkgbuild.com:
     health_check_path: /lastupdate
+  riscv.mirror.pkgbuild.com:
+    health_check_path: /.status/lastupdate.txt