Disable signing for nightlies

[Jaifroid]

Fixes #642.

[Jaifroid]

It was so hard to get electron-builder to sign correctly in the first place, you'd have thought it would be easy to disable signing. Nothing as simple as a switch, oh no. Here's what worked eventually:

• Created a custom configuration file (electronBuilder.cjs) that:
  • Loads the base configuration from package.json
  • Removes all signing-related properties (certificateSha1, signingHashAlgorithms, rfc3161TimeStampServer, and sign) from the win object
  • Unsets top-level signing configs (cscLink and cscKeyPassword)

This is loaded via two -unsigned package.json scripts that are called conditionally.