_1password-gui: update-sources.py use Sequoia instead of GnuPG

r-ryantm update environment fails to go through GnuPG backed PGP
signature verification due to various intricacies of GnuPG's IPC
requirements to do rather basic things.

See: https://nixpkgs-update-logs.nix-community.org/_1password-gui-beta/2025-08-20.log

Switch from GnuPG to Sequoia, and use `sq` CLI tool for a more
minimalistic signature verification.

This change also simplifies trust anchor key acquisition to downloading
directly from 1Password's HTTPS service, instead of fetching by key id
from a PGP Key Server.

Testing:

  % for pkg in _1password-gui{,-beta}; do
   nix-shell ~nixpkgs/maintainers/scripts/update.nix --argstr commit true --argstr skip-prompt true --argstr commit true --argstr package ${pkg}
  done

  Going to be running update for following packages:
   - 1password-8.11.11

  Running update for:
  Preparing worktree (new branch 'update-tmp_manrfba')
  Updating files: 100% (49124/49124), done.
  HEAD is now at 1c8e629a6193 _1password-gui: update-sources.py use Sequoia
  Enqueuing group of 1 packages
   - 1password-8.11.11: UPDATING ...
  [master 6228d7811fce] _1password-gui: 8.11.11 -> 8.11.14
   Date: Sun Oct 19 22:42:25 2025 +0000
   1 file changed, 10 insertions(+), 10 deletions(-)
  Deleted branch update-tmp_manrfba (was 6228d7811fce).

  Packages updated!

  Going to be running update for following packages:
   - 1password-8.11.14-19.BETA

  Running update for:
  Preparing worktree (new branch 'update-tmpezi_89b8')
  Updating files: 100% (49124/49124), done.
  HEAD is now at 6228d7811fce _1password-gui: 8.11.11 -> 8.11.14
  Enqueuing group of 1 packages
   - 1password-8.11.14-19.BETA: UPDATING ...
  [master cd5f65a2f240] _1password-gui-beta: 8.11.14-19.BETA -> 8.11.16-30.BETA
   Date: Sun Oct 19 22:43:03 2025 +0000
   1 file changed, 10 insertions(+), 10 deletions(-)
  Deleted branch update-tmpezi_89b8 (was cd5f65a2f240).

  Packages updated!

0 files changed, 0 insertions, 0 deletions