Bump to Beta 0.4.0, install from GitHub

- version 0.4.0 in install.sh, .env and the service package.json files
- the installer and its release archives come from GitHub by default;
  REPO_BASE / REPO_FALLBACK / ARCHIVE_URL still override the source
- README: install commands point to raw.githubusercontent.com

awg-agent/package.json

@@ -1,6 +1,6 @@
 {
   "name": "awg-agent",
-  "version": "0.3.4",
+  "version": "0.4.0",
   "private": true,
   "description": "Агент ноды: держит исходящее WSS-соединение к core и исполняет запросы на локальном awg-ctrl",
   "scripts": {