mas-dev build won't run due to invalid code signature

[cwillisf]

Expected Behavior

Though we can't run the mas build locally, we should be able to run the mas-dev build locally in order to test something as close to the mas build as possible.

To some extent this is a developer-facing feature request, but it's important because it prevents locally debugging mas-specific problems like #118

Actual Behavior

The mas-dev build fails to launch with a message indicating an invalid code signature.

Steps to Reproduce

1. Modify scripts/electron-builder-wrapper.js to build mas-dev instead of mas
2. Run npm run dist
3. Attempt to start dist/mas-dev/Scratch Desktop.app

Operating System and Browser

macOS