iosevka: add update script

As outlined in the previous commit, we just need to update the source
URL in node-packages.json, and wait for node-packages.nix to be rebuilt
from it.

pkgs/data/fonts/iosevka/default.nix

@@ -89,6 +89,10 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
+  passthru = {
+    updateScript = ./update-default.sh;
+  };
+
   meta = with stdenv.lib; {
     homepage = "https://be5invis.github.io/Iosevka";
     downloadPage = "https://github.com/be5invis/Iosevka/releases";

pkgs/data/fonts/iosevka/update-default.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash -p common-updater-scripts coreutils gawk replace
+set -euo pipefail
+cd "$(dirname "${BASH_SOURCE[0]}")"
+
+nixpkgs=../../../..
+repo=https://github.com/be5invis/Iosevka
+
+# Discover the latest version.
+current_version=$(nix-instantiate "$nixpkgs" --eval --strict -A iosevka.version | tr -d '"')
+new_version=$(list-git-tags "$repo" | sort --reverse --version-sort | awk 'match($0, /^v([0-9.]+)$/, m) { print m[1]; exit; }')
+if [[ "$new_version" == "$current_version" ]]; then
+    echo "iosevka: no update found"
+    exit
+fi
+
+# Update the source package in nodePackages.
+current_source="$repo/archive/v$current_version.tar.gz"
+new_source="$repo/archive/v$new_version.tar.gz"
+replace-literal -ef "$current_source" "$new_source" ../../../development/node-packages/node-packages.json
+echo "iosevka: $current_version -> $new_version (after nodePackages update)"