Fix C++ `concatStringsSep` #11129

Job	Run time
Check Cachix and Docker secrets present for installer tests	1s
meson_build (ubuntu-latest)	10m 32s
vm_tests	21m 54s
meson_build (macos-latest)	4m 25s
tests (ubuntu-latest)	48m 48s
tests (macos-latest)	34m 52s
installer	0s
docker_push_image	0s
installer_test	0s
	2h 0m 32s

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