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

ci/build/npm-postinstall.sh

@@ -140,7 +140,7 @@ install_with_yarn_or_npm() {
         echo "yarn.lock file present, running in development mode. use yarn to install code-server!"
         exit 1
       else
-        npm install --omit=dev
+        npm install --unsafe-perm --legacy-peer-deps --omit=dev
       fi
       ;;
     *)