doc/rl-2.19: add entry for always-allow-substitutes option (backport #11775) #13009

Job	Run time
Check Cachix and Docker secrets present for installer tests	1s
vm_tests	21m 45s
meson_build (ubuntu-latest)	30s
meson_build (macos-latest)	1m 22s
tests (ubuntu-latest)	29m 54s
tests (macos-latest)	41m 4s
flake_regressions	37m 24s
installer	0s
docker_push_image	0s
installer_test	0s
	2h 12m 0s

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