fix(npm): use correct flags and update version for npm (#5533)

* fix: update npm-postinstall.sh script

Add --legacy-peer-deps to deal with weird npm issue with vscode
dependencies.

See: https://stackoverflow.com/a/66620869/3015595

* fix: use npm in release-standalone

* chore: update package.json

* chore: bump plugin engine for tests

test/unit/node/test-plugin/package.json

@@ -3,7 +3,7 @@
   "name": "test-plugin",
   "version": "1.0.0",
   "engines": {
-    "code-server": "^4.0.1"
+    "code-server": "^4.6.1-1"
   },
   "main": "out/index.js",
   "devDependencies": {