build: do not print the default value of --c++-standard in help output #604

Job	Run time
Test with C++20 modules enabled / test	12m 20s
Test (clang++, C++20, dev) / test	4m 11s
Test (clang++, C++20, debug) / test	7m 7s
Test with DPDK enabled / test	11m 15s
Test (clang++, C++20, release) / test	4m 29s
Test (clang++, C++23, debug) / test	7m 11s
Test (clang++, C++23, release) / test	15m 32s
Test (clang++, C++23, dev) / test	4m 42s
Test (g++, C++20, debug) / test	10m 21s
Test (g++, C++20, dev) / test	4m 7s
Test (g++, C++20, release) / test	5m 12s
Test (g++, C++23, debug) / test	9m 39s
Test (g++, C++23, dev) / test	4m 6s
Test (g++, C++23, release) / test	4m 59s
	1h 45m 11s

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