Revert "Update to latest upstream nodejs LTS release 20.9.0"

This reverts commit 2e0d58f since npm is broken
behind proxies nodejs/node#50376 and so pyright fails
to build on Launchpad.

snapcraft.yaml

@@ -87,7 +87,7 @@ parts:
     source-tag: $SNAPCRAFT_PROJECT_VERSION
     source-subdir: packages/pyright
     build-environment:
-      - NODEJS_VERSION: "v20.9.0"
+      - NODEJS_VERSION: "v18.18.2"
     npm-node-version: $NODEJS_VERSION
     build-packages:
       - pkg-config