ci: Linux build with kernel 6.x added. #595

Job	Run time
7.0/ubuntu-20.04/gcc/default/	1m 7s
7.0/ubuntu-20.04/gcc/static/	1m 10s
3.15/ubuntu-20.04/gcc/default/	1m 13s
3.14/ubuntu-20.04/gcc/default/	1m 18s
7.0/ubuntu-20.04/gcc/static/CMD_CXXFLAGS=-std=c++11	1m 18s
7.0/ubuntu-20.04/clang/default/CMD_CXXFLAGS=-std=c++11	1m 17s
7.0/ubuntu-20.04/gcc/default4.10/	1m 19s
7.0/ubuntu-20.04/gcc/default4.9/	1m 20s
7.0/ubuntu-20.04/gcc-8/default/	1m 8s
7.0/ubuntu-20.04/clang/default/	1m 6s
7.0/ubuntu-latest/gcc/default/	1m 19s
	13m 35s

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