Don't run flush_source if scheduler is not started. #11761

Job	Run time
build_details	2s
build_js	2m 1s
build_opam (4.14.x)	8m 58s
build_opam (5.2.x)	5m 22s
build_no_depopts	6m 39s
lezer_parse	52s
build_win32 (x64)	17m 37s
build_posix (debian_trixie, amd64)	15m 2s
build_posix (debian_bookworm, amd64)	14m 56s
run_tests (@citest)	12m 56s
build_posix (ubuntu_oracular, amd64)	14m 35s
tree_sitter_parse	58s
run_tests (@doctest)	7m 52s
build_posix (ubuntu_noble, amd64)	15m 45s
run_tests (@mediatest)	12m 48s
build_posix (alpine, amd64)	13m 0s
update_doc	12m 8s
fetch_s3_artifacts	19s
build_docker_minimal (amd64)	2m 5s
build_docker_alpine (amd64)	1m 1s
build_docker_alpine_minimal (amd64)	1m 21s
build_docker (amd64)	2m 27s
update_release	0s
build_docker_release-minimal	0s
build_docker_release	0s
	2h 48m 44s

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