CI: Update code signing env var in GitHub Actions

This is for newer electron-builder. This needs to be set, or else the
latest electron-builder won't attempt to sign/notarize for
Pull Requests.

We want to at least try this once, to try signing in this PR, and see
if the latest electron-builder version fixes the signing/notarizing
issue we have been seeing lately from our binaries built on
GitHub Actions?

.github/workflows/build.yml

@@ -79,6 +79,7 @@ jobs:
       # So if the PR is a fork, we will still build, but will not sign.
       env:
         CSC_LINK: ${{ secrets.CSC_LINK }}
+        CSC_FOR_PULL_REQUEST: ${{ secrets.CSC_LINK }}
         CSC_KEY_PASSWORD: ${{ secrets.CSC_KEY_PASSWORD }}
         APPLEID: ${{ secrets.APPLEID }}
         APPLEID_PASSWORD: ${{ secrets.APPLEID_PASSWORD }}