Add devshell with bpftools and bpftop

.envrc

@@ -0,0 +1 @@
+use flake

.gitignore

@@ -1 +1,2 @@
 result
+.direnv

flake.nix

@@ -12,6 +12,9 @@
     in
     {
       packages.dropworld = pkgs.callPackage ./dropworld {};
+      devShells.default = pkgs.mkShell {
+        buildInputs = with pkgs; [bpftools bpftop];
+      };
     }
   );
 }