use nix-filter to filter sources

This prevents nix from rebuilding conduit when files that don't actually
effect the build are changed.

flake.lock

@@ -59,6 +59,21 @@
         "type": "github"
       }
     },
+    "nix-filter": {
+      "locked": {
+        "lastModified": 1705332318,
+        "narHash": "sha256-kcw1yFeJe9N4PjQji9ZeX47jg0p9A0DuU4djKvg1a7I=",
+        "owner": "numtide",
+        "repo": "nix-filter",
+        "rev": "3449dc925982ad46246cfc36469baf66e1b64f17",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "nix-filter",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1705496572,
@@ -80,6 +95,7 @@
         "crane": "crane",
         "fenix": "fenix",
         "flake-utils": "flake-utils",
+        "nix-filter": "nix-filter",
         "nixpkgs": "nixpkgs"
       }
     },

flake.nix

@@ -2,6 +2,7 @@
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs?ref=nixos-unstable";
     flake-utils.url = "github:numtide/flake-utils";
+    nix-filter.url = "github:numtide/nix-filter";
 
     fenix = {
       url = "github:nix-community/fenix";
@@ -17,6 +18,7 @@
     { self
     , nixpkgs
     , flake-utils
+    , nix-filter
 
     , fenix
     , crane
@@ -73,7 +75,14 @@
     in
     {
       packages.default = builder {
-        src = ./.;
+        src = nix-filter {
+          root = ./.;
+          include = [
+            "src"
+            "Cargo.toml"
+            "Cargo.lock"
+          ];
+        };
 
         # This is redundant with CI
         doCheck = false;