Regenerate builder schema upon every devkit update

[just-jeb]

Is your feature request related to a problem? Please describe.

The schema of custom-webpack is generated during the build and it's generated from the base schema that comes from @angular-devkit/build-angular and an extension.
So essentially every time the base schema is updated the derived schema should be regenerated.

Describe alternatives you've considered

• Get package locks back, which will resolve this problem in some way (a new build will be triggered upon every @Angular-devkit update), but it still will require from users to install a new version every time.
• OR
• Generate schemas at the postinstall hook, instead of build time (still requires from users to run install but saves version publish)

Additional context
More context in #924