Make Nix configuration path optional #26

Job	Run time
Nix and Rust checks	2m 37s
Build artifacts / build-artifacts (x86_64-linux, X64-Linux, ubuntu-22.04)	7m 35s
Build artifacts / build-artifacts (aarch64-linux, ARM64-Linux, namespace-profile-default-arm64)	9m 39s
Build artifacts / build-artifacts (x86_64-darwin, X64-macOS, macos-12)	10m 31s
Build artifacts / build-artifacts (aarch64-darwin, ARM64-macOS, macos-latest-xlarge)	5m 36s
Integration test for magic-nix-cache-action (X64-Linux, ubuntu-22.04)	1m 5s
Integration test for magic-nix-cache-action (ARM64-Linux, namespace-profile-default-arm64)	1m 43s
Integration test for magic-nix-cache-action (X64-macOS, macos-12)	3m 21s
Integration test for magic-nix-cache-action (ARM64-macOS, macos-latest-xlarge)	1m 45s
	43m 52s

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