Generate nightly builds and json files (#357)

PR adds a nightly release pipeline.
Since the logic for the nightly build and release differs significantly
from the usual pipeline, I decided to put it in a separate file. The
same applies to the Python script that generates JSON files.

The nightly pipeline runs at 01:00 UTC every day, producing artefacts
and retaining them for 40 days if commits are made the day before it
runs.

cc https://github.com/paritytech/revive/issues/355
cc https://github.com/paritytech/devops/issues/4141

.github/scripts/json_generator.py

@@ -1,3 +1,13 @@
+#!/usr/bin/env python3
+"""
+This script generates JSON files for different platforms based on GitHub release data.
+It fetches release information from a specified GitHub repository and tag,
+parses the release assets, and generates JSON files for each platform with relevant metadata.
+It also handles checksum files and updates a list.json file for each platform.
+It requires the GITHUB_TOKEN environment variable to be set for authentication.
+Usage:
+    python json_generator.py <repo> <tag>
+"""
 import os
 import sys
 import json