fix(ci): update Node.js version from 18 to 20 for vsce compatibility

Resolves undici dependency compatibility issue where vsce packaging
fails with "File is not defined" errors. The vsce tool's cheerio
dependency pulls in undici v7.13.0+ which requires Node.js 20.18.1+
but workflow was using Node.js 18.20.8.

Signed-off-by: Gavin Didrichsen <gavin.didrichsen@gmail.com>

Author: gavindidrichsen

.github/workflows/nightly.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        node-version: [18]
+        node-version: [20]
         os: [ubuntu-latest, windows-latest, macos-latest]
 
     steps: