build(windows): calculate checksums after signing [skip ci]

prepare_checksums.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+
+set -e
+
+npm install -g checksum
+
+sum_file() {
+  if [[ -f "${1}" ]]; then
+    echo "Calculating checksum for ${1}"
+    checksum -a sha256 "${1}" > "${1}".sha256
+    checksum "${1}" > "${1}".sha1
+  fi
+}
+
+cd assets
+
+for FILE in *; do
+  if [[ -f "${FILE}" ]]; then
+    sum_file "${FILE}"
+  fi
+done
+
+cd ..