mkDummySrc: rework to only stub bins if they actually exist #1287

Job	Run time
check-dead-code	29s
check-fmt	43s
examples-linux (alt-registry)	2m 56s
examples-darwin	12m 37s
examples-linux (build-std)	2m 21s
tests (https://nixos.org/nix/install)	14m 11s
examples-linux (cross-musl)	1m 33s
tests (https://releases.nixos.org/nix/nix-2.24.10/install, --override-input nixpkgs $(./ci/ref-fr...	15m 12s
examples-linux (cross-rust-overlay)	1m 51s
examples-linux (cross-windows)	1m 27s
examples-linux (custom-toolchain)	1m 31s
examples-linux (end-to-end-testing)	3m 54s
examples-linux (quick-start)	1m 44s
examples-linux (quick-start-simple)	1m 37s
examples-linux (quick-start-workspace)	2m 37s
examples-linux (sqlx)	5m 56s
examples-linux (trunk-workspace)	3m 20s
examples-linux (trunk)	2m 16s
all systems go	0s
	1h 16m 15s

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