Fix Appveyor CI to use NUT libmodbus fork #2609

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 10s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, no, libusb-0.1)	5m 23s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, no)	4m 38s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, libusb-1.0)	5m 6s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, libusb-0.1)	5m 4s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, no)	4m 41s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, libusb-1.0)	4m 57s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, libusb-0.1)	5m 21s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, no)	4m 44s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, libusb-1.0)	5m 23s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, libusb-0.1)	5m 9s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, no)	4m 45s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, libusb-1.0)	5m 15s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, libusb-0.1)	4m 48s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, no)	4m 57s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, libusb-1.0)	5m 6s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, libusb-0.1)	5m 7s
Analyze (python, ubuntu-latest, PYTHON=python3, no, no)	1m 23s
	1h 31m 20s

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