machines/gentoo: use emerge-webrsync
machines/gentoo: use emerge-webrsync

emerge --sync has trouble verifying the HTTPS certificate of
https://keys.gentoo.org and fails when updating the GPG keys. This issue
has been reported on many places:

* https://www.reddit.com/r/Gentoo/comments/cp5pm0/cannot_run_emerge_sync/
* https://forums.gentoo.org/viewtopic-t-1100202-highlight-.html
* https://forums.gentoo.org/viewtopic-t-1084760-start-0.html
* https://forums.gentoo.org/viewtopic-t-1084746.html

fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared