LocateBuildToolsConfigTask fails when buildtools.json does not exist

[dansiegel]

Description

After installing the Mobile.BuildTools on a freshly created project, the LocateBuildToolsConfigTask fails unexpectedly with a FileNotFoundException.

Reproduction Steps

1. Create new application
2. Install the Mobile.BuildTools

Expected Behavior

The Mobile.BuildTools should create a buildtools.json

Actual Behavior

/Users/dansiegel/.nuget/packages/mobile.buildtools/2.0.199-pre-g9558771bca/build/Mobile.BuildTools.targets(5,5): Error MSB4018: The "LocateBuildToolsConfigTask" task failed unexpectedly.
System.IO.FileNotFoundException: Could not find file "/Users/dansiegel/repos/Temp/MBTTest1/buildtools.json"
File name: '/Users/dansiegel/repos/Temp/MBTTest1/buildtools.json'
  at System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize, System.Boolean anonymous, System.IO.FileOptions options) [0x00259] in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/mono-x64/mcs/class/corlib/System.IO/FileStream.cs:274 
  at System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize, System.IO.FileOptions options) [0x00000] in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/mono-x64/mcs/class/corlib/System.IO/FileStream.cs:106 
  at (wrapper remoting-invoke-with-check) System.IO.FileStream..ctor(string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare,int,System.IO.FileOptions)
  at System.IO.StreamReader..ctor (System.String path, System.Text.Encoding encoding, System.Boolean detectEncodingFromByteOrderMarks, System.Int32 bufferSize) [0x00055] in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/mono-x64/external/corefx/src/Common/src/CoreLib/System/IO/StreamReader.cs:182 
  at System.IO.StreamReader..ctor (System.String path, System.Text.Encoding encoding, System.Boolean detectEncodingFromByteOrderMarks) [0x00000] in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/mono-x64/external/corefx/src/Common/src/CoreLib/System/IO/StreamReader.cs:167 
  at (wrapper remoting-invoke-with-check) System.IO.StreamReader..ctor(string,System.Text.Encoding,bool)
  at System.IO.File.InternalReadAllText (System.String path, System.Text.Encoding encoding) [0x00000] in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/mono-x64/external/corefx/src/System.IO.FileSystem/src/System/IO/File.cs:303 
  at System.IO.File.ReadAllText (System.String path) [0x00026] in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/mono-x64/external/corefx/src/System.IO.FileSystem/src/System/IO/File.cs:282 
  at Mobile.BuildTools.Utils.ConfigHelper.GetConfig (System.String path) [0x00044] in D:\a\1\s\src\Mobile.BuildTools.Reference\Utils\ConfigHelper.shared.cs:63 
  at Mobile.BuildTools.Tasks.LocateBuildToolsConfigTask.Execute () [0x00033] in D:\a\1\s\src\Mobile.BuildTools\Tasks\LocateBuildToolsConfigTask.cs:67 
  at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute () [0x00023] in /Users/builder/jenkins/workspace/build-package-osx-mono-pullrequest/pr/external/bockbuild/builds/msbuild-15/src/Build/BackEnd/TaskExecutionHost/TaskExecutionHost.cs:578 
  at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask (Microsoft.Build.BackEnd.ITaskExecutionHost taskExecutionHost, Microsoft.Build.BackEnd.Logging.TaskLoggingContext taskLoggingContext, Microsoft.Build.BackEnd.TaskHost taskHost, Microsoft.Build.BackEnd.ItemBucket bucket, Microsoft.Build.BackEnd.TaskExecutionMode howToExecuteTask) [0x002b9] in /Users/builder/jenkins/workspace/build-package-osx-mono-pullrequest/pr/external/bockbuild/builds/msbuild-15/src/Build/BackEnd/Components/RequestBuilder/TaskBuilder.cs:807  (MSB4018) (MBTTest1)

Environment

• OS: [Windows 10|OSX]
• IDE: [VS2017|VSMac|CLI]
• BuildTools Version: [1.x.x]
• IDE Version: [1.x.x]
• Build Target Framework: [netstandard2.0|Xamarin.iOS|Xamarin.Android]

Reproduction App

n/a