linux_zen: add update script

pkgs/os-specific/linux/kernel/update-zen.sh

@@ -0,0 +1,21 @@
+#! /usr/bin/env nix-shell
+#! nix-shell -I nixpkgs=../../../.. -i bash -p nix-prefetch git gnused gnugrep nix curl
+set -euo pipefail -x
+
+nixpkgs="$(git rev-parse --show-toplevel)"
+old=$(nix-instantiate --eval -A linuxPackages_zen.kernel.modDirVersion "$nixpkgs")
+old="${old%\"}"
+old="${old#\"}"
+new=$(curl https://github.com/zen-kernel/zen-kernel/releases.atom | grep -m1 -o -E '[0-9.]+-zen[0-9]+')
+if [[ "$new" == "$old" ]]; then
+    echo "already up-to-date"
+    exit 0
+fi
+
+path="$nixpkgs/pkgs/os-specific/linux/kernel/linux-zen.nix"
+
+sed -i -e "s!modDirVersion = \".*\"!modDirVersion = \"${new}\"!" "$path"
+checksum=$(nix-prefetch "(import ${nixpkgs} {}).linuxPackages_zen.kernel")
+sed -i -e "s!sha256 = \".*\"!sha256 = \"${checksum}\"!" "$path"
+
+git commit -m "linux_zen: ${old} -> ${new}" $path