home-manager: check FQDN for '--flake .' attribute

Since nixpkgs commit 993baa587c4b82e791686f6ce711bcd4ee8ef933,
`networking.hostName` is not allowed to be a FQDN.

Add `hostname --fqdn` to the default flake attribute names that are
searched.

If

    netorking.hostname = "hostname";
    networking.domain = "example.com";

is set in the system NixOS configuration, this allows defining

    homeConfigurations."username@hostname.example.com" = ...

and still use

    home-manager switch --flake .

instead of having to type out

    home-manager switch --flake .#$(whoami)@$(hostname --fqdn)

home-manager/home-manager

@@ -198,9 +198,9 @@ function setFlakeAttribute() {
                 ;;
             *)
                 local name="$USER"
-                # Check both long and short hostnames; long first to preserve
+                # Check FQDN, long, and short hostnames; long first to preserve
                 # pre-existing behaviour in case both happen to be defined.
-                for n in "$USER@$(hostname)" "$USER@$(hostname -s)"; do
+                for n in "$USER@$(hostname --fqdn)" "$USER@$(hostname)" "$USER@$(hostname -s)"; do
                     if [[ "$(nix eval "$flake#homeConfigurations" --apply "x: x ? \"$n\"")" == "true" ]]; then
                         name="$n"
                         if [[ -v VERBOSE ]]; then