Remove redundant signing determination code from build system

The "build" workflow signs the macOS and Windows builds of the application. The signing process relies on access to GitHub Actions
secrets. For this reason, the workflow is configured to only sign the builds when it has access to GitHub Actions
secrets to avoid spurious failures of the workflow that would otherwise be caused by signing failure.

A flexible general purpose system for determining whether to attempt signing of a build was established years ago. However, a redundant system was added specific to the Windows build instead of using the existing system.

The redundant system is hereby removed. This makes the workflow easier to understand and maintain.

Author: per1234

.github/workflows/build.yml

@@ -290,7 +290,6 @@ jobs:
       SIGNTOOL_PATH: "C:/Program Files (x86)/Windows Kits/10/bin/10.0.19041.0/x86/signtool.exe"
       WIN_CERT_PASSWORD: ${{ secrets[matrix.config.certificate-password-secret] }}
       WIN_CERT_CONTAINER_NAME: ${{ secrets[matrix.config.certificate-container] }}
-      WIN_SIGNING_ENABLED: ${{ secrets[matrix.config.certificate-password-secret] != '' }}
 
     strategy:
       matrix:

electron-app/scripts/windowsCustomSign.js

@@ -1,10 +1,7 @@
 const childProcess = require('child_process');
 
 exports.default = async function (configuration) {
-  if (
-    !process.env.GITHUB_ACTIONS ||
-    process.env.WIN_SIGNING_ENABLED !== 'true'
-  ) {
+  if (!process.env.GITHUB_ACTIONS || process.env.CAN_SIGN !== 'true') {
     return;
   }