Fix Appveyor CI to use NUT libmodbus fork #2602

Job	Run time
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, no, no)	4m 23s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, no, libusb-1.0)	5m 2s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, no, libusb-0.1)	4m 58s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, no)	5m 1s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, libusb-1.0)	4m 40s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, libusb-0.1)	4m 42s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, no)	5m 26s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, libusb-1.0)	4m 52s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, libusb-0.1)	4m 35s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, no)	5m 3s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, libusb-1.0)	4m 54s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, libusb-0.1)	6m 22s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, no)	4m 37s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, libusb-1.0)	6m 20s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, libusb-0.1)	5m 21s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, no)	5m 57s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, libusb-1.0)	5m 0s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, libusb-0.1)	5m 25s
Analyze (python, ubuntu-latest, PYTHON=python3, no, no)	1m 7s
	1h 33m 45s

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