Fix Appveyor CI to use NUT libmodbus fork #2611

Job	Run time
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, no, no)	4m 35s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, no, libusb-1.0)	5m 49s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, no, libusb-0.1)	4m 47s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, no)	4m 46s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, libusb-1.0)	4m 45s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, libusb-0.1)	8m 28s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, no)	4m 38s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, libusb-1.0)	4m 53s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, libusb-0.1)	5m 1s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, no)	4m 45s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, libusb-1.0)	4m 59s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, libusb-0.1)	5m 7s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, no)	5m 33s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, libusb-1.0)	5m 14s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, libusb-0.1)	5m 18s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, no)	4m 44s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, libusb-1.0)	5m 30s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, libusb-0.1)	5m 32s
Analyze (python, ubuntu-latest, PYTHON=python3, no, no)	1m 8s
	1h 35m 32s

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