[Embroider] Fastboot enabled apps might fail under embroider-optimized

[BobrImperator]

Encountered while working on ember-simple-auth. ESA test-suite tests some basic scenarios with fastboot in one of the test apps.
embroider-optimized scenario fails. See Embroider Optimized Test Scenario

The error points to this file:
https://github.com/ember-fastboot/ember-cli-fastboot/blob/master/packages/ember-cli-fastboot/vendor/experimental-render-mode-rehydrate.js#L4

ReferenceError: Ember is not defined
    <anonymous> experimental-render-mode-rehydrate.js:4
    <anonymous> experimental-render-mode-rehydrate.js:33

embroider-safe scenario works just fine.

mainmatter/ember-simple-auth#2633 (comment)

[villander]

I have the same error using the following packages

 "@embroider/compat": "^3.4.5",
 "@embroider/core": "^3.4.5",
 "@embroider/webpack": "^3.2.2",
 "ember-cli-fastboot": "^4.1.2",

cc @mansona