Fix nix update scripts to work without using a system-wide nixFlake

nix/README.md

@@ -0,0 +1,19 @@
+# Nix dependencies
+
+The nix build use the new flake format to manage dependencies. A flake-compatible nix command is provided from within `nix-shell`. To add flake support to your native nix setup please see https://nixos.wiki/wiki/Flakes.
+
+Cardano-node nix build depends primarily on [haskell.nix](https://github.com/input-output-hk/haskell.nix) and secondarily, for some utilities, on [iohk-nix](https://github.com/input-output-hk/iohk-nix/).
+
+Both can be updated from within a cardano-node `nix-shell` with:
+
+```
+nix flake update --update-input haskellNix
+nix flake update --update-input iohkNix
+```
+
+Or from outside the `nix-shell` with the scripts:
+
+```
+./nix/update-haskellNix.sh
+./nix/update-iohkNix.sh
+```

nix/update-haskellNix.sh

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+cd "$(dirname "$0")/.."
+nix-shell -I nixpkgs=./nix -p nixWrapped \
+  --run "nix flake update --update-input haskellNix"

nix/update-iohkNix.sh

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+cd "$(dirname "$0")/.."
+nix-shell -I nixpkgs=./nix -p nixWrapped \
+  --run "nix flake update --update-input iohkNix"