Fix C++ `concatStringsSep` #11138

Job	Run time
Check Cachix and Docker secrets present for installer tests	1s
vm_tests	21m 43s
meson_build (ubuntu-latest)	10m 25s
meson_build (macos-latest)	4m 21s
tests (ubuntu-latest)	48m 48s
tests (macos-latest)	30m 26s
installer	0s
docker_push_image	0s
installer_test	0s
	1h 55m 44s

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