flake(goModule): filter out non-pertinent files...

...such as flake files or {shell,default}.nix to reduce the necessity to
rebuild (when using `nix build`) after each change, even to non-go
files. more files can be added to an exclude list in the future.

flake.lock

@@ -16,6 +16,21 @@
         "type": "github"
       }
     },
+    "nix-filter": {
+      "locked": {
+        "lastModified": 1653590866,
+        "narHash": "sha256-E4yKIrt/S//WfW5D9IhQ1dVuaAy8RE7EiCMfnbrOC78=",
+        "owner": "numtide",
+        "repo": "nix-filter",
+        "rev": "3e81a637cdf9f6e9b39aeb4d6e6394d1ad158e16",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "nix-filter",
+        "type": "github"
+      }
+    },
     "nixgl": {
       "inputs": {
         "nixpkgs": [
@@ -55,6 +70,7 @@
     "root": {
       "inputs": {
         "flake-compat": "flake-compat",
+        "nix-filter": "nix-filter",
         "nixgl": "nixgl",
         "nixpkgs": "nixpkgs"
       }

flake.nix

@@ -10,9 +10,13 @@
       flake = false;
       inputs.nixpkgs.follows = "nixpkgs";
     };
+    nix-filter = {
+      url = "github:numtide/nix-filter";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
-  outputs = { self, nixgl, nixpkgs, ... }:
+  outputs = { self, nixgl, nix-filter, nixpkgs, ... }:
     let
 
       # to work with older version of flakes
@@ -87,7 +91,15 @@
 
             # In 'nix develop', we don't need a copy of the source tree
             # in the Nix store.
-            src = lib.cleanSource ./.;
+            src = nix-filter.lib.filter {
+              root = lib.cleanSource ./.;
+              exclude = [
+                ./flake.nix
+                ./flake.lock
+                ./default.nix
+                ./shell.nix
+              ];
+            };
 
             meta = {
               description = "an offline-capable xkcd webcomic reader written in Go";