home-manager: fix early exit due to FQDN error

Since 89670e27e1, FQDN lookup errors
from `hostname -f` may cause an early exit of the whole program.

This commit fixes the problem by absorbing the FQDN query inside the
`if` statement.

home-manager/home-manager

@@ -202,8 +202,7 @@ function setFlakeAttribute() {
                 local hostnameArray=()
                 # FQDN lookup can fail depending on the resolver.
                 local fqdn
-                fqdn="$(hostname -f 2> /dev/null)"
-                if [[ $? -eq 0 ]]; then
+                if fqdn="$(hostname -f 2> /dev/null)"; then
                     hostnameArray+=( "$USER@$fqdn"  )
                 fi
                 # Check FQDN, long, and short hostnames; long first to preserve