feat(ios): always override xcconfig with user values

[edusperoni]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• Tests for the changes are included.

What is the current behavior?

Values in the user's build.xcconfig are sometimes concatenated to the base values from the iOS template

What is the new behavior?

build.xcconfig always takes precedence over default values (no merging)