Revert "Update to latest upstream nodejs LTS release 20.9.0"

This reverts commit 03a0cd2 since npm is broken
behind proxies nodejs/node#50376 and so copilot-client
fails to build on Launchpad.

snapcraft.yaml

@@ -33,7 +33,7 @@ parts:
     source: https://github.com/github/copilot.vim.git
     source-tag: v$SNAPCRAFT_PROJECT_VERSION
     build-environment:
-      - NODEJS_VERSION: "v20.9.0"
+      - NODEJS_VERSION: "v18.18.2"
     npm-node-version: $NODEJS_VERSION
     override-build: |
       # install the official nodejs so we get the expected npm version