[cloud_functions] Use correct method channel

[cbenhagen]

Description

Unfortunately the integration test did not catch this.

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[mockturtl]

@cbenhagen Do you happen to know the last working version I can use until this lands?

[cbenhagen]

0.4.1+9 should work

[mockturtl]

0.4.1+9 should work

Confirmed. Thanks!

[collinjackson]

There's a test that should have caught this:

https://github.com/FirebaseExtended/flutterfire/blob/master/packages/cloud_functions/cloud_functions/example/test/cloud_functions.dart

This should be caught by our CI, issue is already filed #1870