Fix Appveyor CI to use NUT libmodbus fork #2605

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)	4m 46s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, no, libusb-0.1)	4m 55s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, no)	4m 44s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, libusb-1.0)	4m 50s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, libusb-0.1)	4m 56s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, no)	4m 39s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, libusb-1.0)	4m 56s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, libusb-0.1)	4m 44s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, no)	4m 43s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, libusb-1.0)	4m 53s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, libusb-0.1)	6m 23s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, no)	5m 5s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, libusb-1.0)	6m 19s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, libusb-0.1)	4m 58s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, no)	4m 50s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, libusb-1.0)	6m 27s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, libusb-0.1)	5m 1s
Analyze (python, ubuntu-latest, PYTHON=python3, no, no)	1m 8s
	1h 32m 52s

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