Define the initial NixOS configuration of webforge in a flake with CI support

[btlogy]

Duplicates #32 which implements #31 and adds support for CI with 99 extra LoC, which partly implements #34.

The CI supports is actually automating the 3 first commands described in #31:

nix flake check
nix build .#nixosConfigurations.webforge.config.system.build.toplevel
nixos-rebuild dry-activate --flake .#webforge --target-host root@webforge.tahoe-lafs.org

The automated deployment will come later (with a lot more LoC).

[btlogy]

@hacklschorsch was right: the initial way this PR was dealing with nix eval to dynamically create a matrix job was over-complicated for only a few host in this project...
We might find a better way to re-introduce this later (if ever needed).
But for now, we just repeat the sysname, hostname and domainname in nix and in GHA, and that's fine if it is easier to maintain.