Environment variables are not replaced in resulting Info.plist

# Description
Environment variables are not replaced in Info.plist
I bet it is related to #284

## Reproduction Steps

1. Create new Xamarin.Forms app
2. Install Mobile.BuildTools (leave default buildtools.json)
3. Define

` "environment": {
"defaults": {
"AppId": "com.something.not.working" }
},`

4. Set to Debug, Simulator
5. Build

## Expected Behavior

Variable $AppId$ is replaced with "com.something.not.working" in "\obj\iPhoneSimulator\Debug\mbt" folder

## Actual Behavior

Variable is not replaced

## Environment

- OS: [Windows 10|
- IDE: [VS2019|
- BuildTools Version: [2.x.x]
- IDE Version: [16.11.3]
- Build Target Framework: [netstandard2.0|Xamarin.iOS|