docs: add FlakeHub badge and update input URL #1

Job	Run time
nix-ci / inventory	1m 0s
nix-ci / build (aarch64-linux, ubuntu-24.04-arm)	4s
nix-ci / build (aarch64-darwin, macos-latest)	16s
nix-ci / build (x86_64-linux, ubuntu-latest)	9s
nix-ci / success	5s
	1m 34s

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