feat(minion): use vivaldi browser #110

I've gotten fed up of Firefox's AI/user unfriendliness and Thunderbird's
broken calendaring, specifically
- Thunderbird, despite being a mail client, cannot seem to send calendar
invites to people. Shockingly, Android's builtin calendar (which is not
a mail client) seems to do a better job at this(!!)
- Firefox seems to be adding more and more AI features which are annoying
to opt out of. I can't be bothered to write the nix config to disable
these as a matter of policy (and I really shouldn't have to). In my
opinion, investing in these features shows that FF is out-of-touch with
its userbase and no longer cares about making a good browsing
experience
- Firefox is behind on important web standards, in my case particularly
WebUSB and Bluetooth (used for some hardware I have)
- Firefox makes decisions which give webmasters the final say rather than
me, the user. Specifically, Firefox has no way to override HSTS
(Chromium lets 'thisisunsafe' override HSTS). This is not only a bad
move on part of user freedom, it's also *really annoying* for
development
- Chromium also makes some of these decisions, *cough ManifestV3*, and
Google attempts to make more, *cough Web Environment Integrity*, but
Vivaldi comes with a built-in Adblocker and I can - more or less -
get everything I need on a Chromium based browser

Vivaldi fixes both of these, at the cost of being proprietary. That means
- We can't build it in CI
- We definitely can't make it default (perhaps it could be an ingredient)

We can't remove the other browsers we have since as we commonly need them
for testing

If anyone has any good FOSS alternatives that I've missed, I'd love to
hear about them... For my purposes, LibreWolf is not a *good* FOSS
alternative (it makes various decisions that break websites), neither is
base Chromium.