[nix] Add to version bumping script

* Update generate_packages.sh structure to split its code to separate
  steps
* Add update_flake to generate_packages script
* Add generate_packages package to nix packages to be able to run
  `nix run .#generate_packages` with no need to install dependencies
  manually

Change-Id: Ic28107829a6e04d3831ccb8a7257164875f35938

nix/generate_packages.nix

@@ -0,0 +1,25 @@
+{ jq
+, curl
+, runCommand
+, lib
+, makeWrapper
+}:
+let
+  paths = lib.makeBinPath [ jq curl ];
+in
+runCommand "generate_config"
+{
+  src = ../generate_packages.sh;
+  name = "generate_packages";
+  nativeBuildInputs = [
+    makeWrapper
+  ];
+} ''
+  mkdir -p "$out/bin"
+  bin="$out/bin/$name"
+  cp "$src" "$bin"
+  chmod +x "$bin"
+  patchShebangs "$bin"
+  wrapProgram "$bin" \
+    --prefix PATH : ${paths}
+''