Revert "linux-builder: remove DNS hack for libslirp"

Resolution still fails when on VPN with no IPv6 DNS servers. We'll need
to investigate further why the fix doesn't help in this case.

This reverts commit f90236a8f2.

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

nixos/modules/profiles/nix-builder-vm.nix

@@ -111,6 +111,14 @@ in
       };
     };
 
+    # DNS fails for QEMU user networking (SLiRP) on macOS.  See:
+    #
+    # https://github.com/utmapp/UTM/issues/2353
+    #
+    # This works around that by using a public DNS server other than the DNS
+    # server that QEMU provides (normally 10.0.2.3)
+    networking.nameservers = [ "8.8.8.8" ];
+
     # The linux builder is a lightweight VM for remote building; not evaluation.
     nix.channel.enable = false;