Skip local prepare when executing cloud build from sidekick and no project's file is changed

[Fatme]

Platform prepare is never skipped when executing cloud build for iOS from sidekick. The reason for this is the signingChanged property from changesInfo is always true and thus shouldPrepare is always true. signingChanged property is set to true inside ios-project-service because pbxproject file does not exist. Actually locally from sidekick pbxproject file is never created. So the file never exists. This PR speeds the the cloud build for iOS with around 2-3 sec.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
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• Tests for the changes are included.

What is the current behavior?

Local prepare is not skipped when no project's file is changed.

What is the new behavior?

Local prepare is skipped when no project's file is changed.