mkDummySrc: dedup discovered and declared bin targets #1290

Job	Run time
examples-darwin	10m 11s
check-fmt	37s
check-dead-code	22s
examples-linux (alt-registry)	2m 54s
examples-linux (build-std)	2m 9s
tests (https://nixos.org/nix/install)	14m 35s
examples-linux (cross-musl)	1m 34s
tests (https://releases.nixos.org/nix/nix-2.24.10/install, --override-input nixpkgs $(./ci/ref-fr...	14m 55s
examples-linux (cross-rust-overlay)	1m 53s
examples-linux (cross-windows)	1m 27s
examples-linux (custom-toolchain)	1m 37s
examples-linux (end-to-end-testing)	4m 6s
examples-linux (quick-start)	1m 46s
examples-linux (quick-start-simple)	1m 41s
examples-linux (quick-start-workspace)	2m 37s
examples-linux (sqlx)	4m 17s
examples-linux (trunk-workspace)	3m 22s
examples-linux (trunk)	2m 19s
all systems go	0s
	1h 12m 22s

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