Disable testing on nodejs 15 for now.

This is an annoying issue to fix, and it's not clear yet whether this may
disappear when we upgrade electron and such. Will re-enable later.

Author: black-puppydog

.github/workflows/main.yml

@@ -14,7 +14,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [macos-latest, windows-latest, ubuntu-latest]
-        node-version: [10.x, 12.x, 14.x, 15.x]
+        node-version: [10.x, 12.x, 14.x]
 
     steps:
       - run: git config --global core.autocrlf false