fix(suite-desktop): macOS ARM build adhoc signing

[Lemonexe]

Description

Followup to #18819, which broke CI dev desktop build for macOS 🍎

With electron-builder 26.0.13 a change was done in #9007:

• if identity ENV is missing (as in dev builds) ARM builds are now adhoc signed by default
  • docs here
• that crashes in CI
• so explicitly skip signing when not codesign build

🔍🖥️ Suite desktop test results: View in Currents

[vojtatranta]

I think there is a problem:

• for some reason, there isn't an icon of the Trezor Suite app, but it was in the previous version of the electron-builder
• now, x64 build WON'T be started on Mac but it WAS possible to be started on the previous version of the electron builder, but it was likely because I made a build with identity: '-' which triggered ad-hoc signing

[vojtatranta]

And local build x64 won't start at all with error: https://pastebin.com/u6GgqPSU