Add: script for update flake lock

Take commands out of the `action.yml` file, and put it in a dedicated
shell script.

update-flake-lock.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ -n "$TARGETS" ]]; then
+    inputs=()
+    for input in $TARGETS; do
+        inputs+=("--update-input" "$input")
+    done
+    nix flake lock "${inputs[@]}" --commit-lock-file --commit-lockfile-summary "$COMMIT_MSG"
+else
+    nix flake update --commit-lock-file --commit-lockfile-summary "$COMMIT_MSG"
+fi