installation should keep existing postinstall scripts

[tmtron]

Describe the bug
when we install the electron-builder package it will overwrite existing postinstall scripts in package.json

To Reproduce

1. create nx workspace project: npx create-nx-workspace --name=nx-electron-tmp --preset=empty --appName=app --nx-cloud=0
2. cd nx-electron-tmp/
3. add a postinstall script to package.json: e.g. "postinstall": "echo existing-postinstall"
4. install nx-electron: npm install -D nx-electron
5. create nx-electron app: nx g nx-electron:app
   this will overwrite any previous postinstall scripts

Expected behavior
the installation should only append to an existing postinstall script

e.g. "postinstall": "echo existing-postinstall && exitzero electron-builder install-app-deps"

Desktop (please complete the following information):

• OS: Windows 10 64-bit
• Nx Electron Version : 11.1.1

Additional context
This is a minor issue, because it is usually easy to see the changes when we commit to git.