Fix more licenses (#670)

* Move optional steps for testing VMs out of README.md file
This may help avoid new users being confused that they must follow
those instructions.

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

* Add 2025-Jan-30 versions of source code for install.sh

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

* Correct licenses on two test programs that use Scapy indirectly

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

---------

Signed-off-by: Andy Fingerhut <andy_fingerhut@alum.wustl.edu>

Author: jafingerhut

exercises/link_monitor/receive.py

@@ -1,5 +1,6 @@
 #!/usr/bin/env python3
-# SPDX-License-Identifier: Apache-2.0
+# SPDX-License-Identifier: GPL-2.0-only
+# Reason-GPL: import-scapy via probe_hdrs
 
 from probe_hdrs import *
 

exercises/link_monitor/send.py

@@ -1,5 +1,6 @@
 #!/usr/bin/env python3
-# SPDX-License-Identifier: Apache-2.0
+# SPDX-License-Identifier: GPL-2.0-only
+# Reason-GPL: import-scapy via probe_hdrs
 import sys
 import time