CI configs: add nix flake #1789

Job	Run time
Run tests (emulated riscv64)	1m 3s
Run tests (ubuntu, 1.23.x)	1m 35s
Run tests (emulated s390x)	1m 11s
Run tests (macos, 1.23.x)	1m 42s
Run tests (windows, 1.23.x)	4m 36s
Run benchmarks	1m 24s
Run checks (ubuntu)	41s
Run tests (ubuntu, 1.22.x)	28s
Run checks (macos)	23s
Check relative links in website	9s
Run checks (windows)	3m 20s
Upload test coverage (linux)	12s
Upload test coverage (macos)	13s
Upload test coverage (windows)	13s
	17m 10s

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