add envrc and shell.nix with 'just' pkg

shell.nix

@@ -0,0 +1,34 @@
+with import <nixpkgs> {};
+
+let 
+  just = rustPlatform.buildRustPackage rec {
+    name = "just-${version}";
+    version = "git";
+    src = builtins.fetchGit {
+      url = "https://github.com/casey/just.git";
+      rev = "6430d38a6a489ba4e818b6a892e8bbbbacbfe8f3";
+    };
+
+    cargoSha256 = "0awfq9fhcin2q6mvv54xw6i6pxhdp9xa1cpx3jmpf3a6h8l6s9wp";
+
+    doCheck = false;
+
+    meta = with stdenv.lib; {
+      description = "Just a command runner ";
+      homepage = https://github.com/casey/just;
+      license = licenses.unlicense;
+      maintainers = [ ];
+      platforms = platforms.all;
+    };
+  };
+
+in 
+stdenv.mkDerivation {
+  name = "infra-env";
+  buildInputs = [
+    just
+  ];
+
+  # Set Environment Variables
+  RUST_BACKTRACE = 1;
+}