mkDummySrc: rework to only stub bins if they actually exist (#752) #1288

Job	Run time
check-dead-code	24s
examples-darwin	12m 38s
examples-linux (alt-registry)	3m 2s
check-fmt	41s
tests (https://nixos.org/nix/install)	13m 43s
tests (https://releases.nixos.org/nix/nix-2.24.10/install, --override-input nixpkgs $(./ci/ref-fr...	14m 18s
examples-linux (build-std)	2m 9s
examples-linux (cross-musl)	1m 42s
examples-linux (cross-rust-overlay)	1m 58s
examples-linux (cross-windows)	1m 27s
examples-linux (custom-toolchain)	1m 31s
examples-linux (end-to-end-testing)	4m 2s
examples-linux (quick-start)	1m 42s
examples-linux (quick-start-simple)	1m 42s
examples-linux (quick-start-workspace)	2m 38s
examples-linux (sqlx)	4m 19s
examples-linux (trunk-workspace)	3m 17s
examples-linux (trunk)	2m 15s
all systems go	0s
	1h 13m 28s

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