home-manager: ignore hostname -f lookup errors

`hostname -f` could fail depending on the resolver. Discard any stderr
and test for the exit status before using the value for flake attribute
lookup.

I was unable to repro the exact bad exit status in #5665.
With
  - nscd disabled,
  - nsswitch.conf pointing to 'files',
  - hostname entry removed from /etc/hosts
`hostname -f` from inetutils-2.5 fell back to showing just the nodename
from `uname(2)`. Injecting an empty string into the
`(struct utsname).nodename` field of `uname(2)` using strace still
exited with empty output and 0 exit-status.

Fixes #5665

home-manager/home-manager

@@ -198,9 +198,19 @@ function setFlakeAttribute() {
                 ;;
             *)
                 local name="$USER"
+
+                local hostnameArray=()
+                # FQDN lookup can fail depending on the resolver.
+                local fqdn
+                fqdn="$(hostname -f 2> /dev/null)"
+                if [[ $? -eq 0 ]]; then
+                    hostnameArray+=( "$USER@$fqdn"  )
+                fi
                 # Check FQDN, long, and short hostnames; long first to preserve
                 # pre-existing behaviour in case both happen to be defined.
-                for n in "$USER@$(hostname -f)" "$USER@$(hostname)" "$USER@$(hostname -s)"; do
+                hostnameArray+=( "$USER@$(hostname)" "$USER@$(hostname -s)" )
+
+                for n in "${hostnameArray[@]}"; do
                     if [[ "$(nix eval "$flake#homeConfigurations" --apply "x: x ? \"$n\"")" == "true" ]]; then
                         name="$n"
                         if [[ -v VERBOSE ]]; then