fix: nix profile circular symlink, mdns hang, deprecated install command

- first-login.sh: remove nix-env --switch-profile (caused .nix-profile->
  .nix-profile circular symlink, breaking all nix profile commands and
  causing ELOOP on any exec via nix PATH including xz/tar/node)
- first-login.sh: add circular symlink guard before nix profile add
- first-login.sh: nix profile install -> nix profile add (deprecated alias)
- live-setup.sh: strip mdns from nsswitch.conf hosts line at boot (no
  libnss_mdns/Avahi in live; caused first-login DNS hang)
- docs/LIVE_ISO.md: document all three issues and their fixes

iso/build-live-iso.sh

@@ -121,6 +121,15 @@ NIXSH
     echo "live-setup: nix daemon mode configured (trusted-users = $LIVE_USER)"
 fi
 
+# ── nsswitch: remove mdns (library absent on Void; no Avahi daemon) ──────
+# 'mdns' in nsswitch.conf without libnss_mdns causes hangs on every DNS
+# lookup because glibc spins waiting for Avahi's socket.  Remove it so
+# standard DNS resolution is used directly.
+if [ -f /etc/nsswitch.conf ]; then
+    sed -i '/^hosts:/s/mdns[^ ]* *//g' /etc/nsswitch.conf
+    echo "live-setup: removed mdns from nsswitch.conf (hosts line)"
+fi
+
 # ── D-Bus session socket for the live user's login session ───────────────
 # dbus-launch is called by lightdm-session / Xsession automatically.
 # Ensure dbus system bus is accessible (needed by nix and Cinnamon).