feat: specify ourselves as Nix flake source

This allows users to directly use the criu package declared in the flake
as a bleeding-edge substitute to the one found in Nixpkgs, and leverage
the existing NixOS testing infrastructure with their development version.

Signed-off-by: Karim Vergnes <me@thesola.io>

flake.lock

@@ -20,11 +20,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1744463964,
-        "narHash": "sha256-LWqduOgLHCFxiTNYi3Uj5Lgz0SR+Xhw3kr/3Xd0GPTM=",
+        "lastModified": 1772624091,
+        "narHash": "sha256-QKyJ0QGWBn6r0invrMAK8dmJoBYWoOWy7lN+UHzW1jc=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "2631b0b7abcea6e640ce31cd78ea58910d31e650",
+        "rev": "80bdc1e5ce51f56b19791b52b2901187931f5353",
         "type": "github"
       },
       "original": {

flake.nix

@@ -71,7 +71,7 @@
         devShells.default = devShell;
 
         # Build CRIU package as well
-        packages.default = pkgs.criu;
+        packages.default = pkgs.criu.overrideAttrs { src = ./.; };
       }
     );
 }