[ci] disable <XamarinTelemetry/> task (#7275)

Context: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1585820
Context: xamarin/XamarinVS#13271

Visual Studio 17.3.0 introduced a new
[`_WriteTelemetryProperties`][0] target into `Xamarin.Sdk.targets`:

	<Target Name="_WriteTelemetryProperties"
	        DependsOnTargets="_CollectAndroidTelemetryProperties;_CollectIOSTelemetryProperties"
	        AfterTargets="Build"
	        Condition="'$(_WriteTelemetryProperties)' != 'false'">

Unfortunately, this new target uses a new `<XamarinTelemetry/>` task
which fails on CI, which impacts many of our MSBuild unit tests:

	error MSB4018: The "XamarinTelemetry" task failed unexpectedly.
	error MSB4018: System.IO.FileNotFoundException: Could not load file or assembly 'Newtonsoft.Json, Version=13.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed' or one of its dependencies. The system cannot find the file specified.
	error MSB4018: File name: 'Newtonsoft.Json, Version=13.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed'
	error MSB4018:    at Microsoft.VisualStudio.Telemetry.TelemetrySessionSettings.GetFaultEventBucketFilterJson(List`1 bucketFilters)
	error MSB4018:    at Microsoft.VisualStudio.Telemetry.TelemetrySessionSettings.ToString()
	error MSB4018:    at Microsoft.VisualStudio.Telemetry.TelemetrySession.SerializeSettings()
	error MSB4018:    at Xamarin.Common.Tasks.Telemetry.GetSession(IBuildEngine4 buildEngine, String vsTelemetrySession) in D:\a\_work\1\s\src\MSBuild\Xamarin.Common.Tasks\Telemetry.cs:line 62
	error MSB4018:    at Xamarin.Common.Tasks.XamarinTelemetry.Execute() in D:\a\_work\1\s\src\MSBuild\Xamarin.Common.Tasks\XamarinTelemetry.cs:line 31
	error MSB4018:    at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
	error MSB4018:    at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext()
	error MSB4018: WRN: Assembly binding logging is turned OFF.
	error MSB4018: To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1.
	error MSB4018: Note: There is some performance penalty associated with assembly bind failure logging.
	error MSB4018: To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].

It appears that this will be fixed in Visual Studio 17.3.1.

In the meantime, fix our CI by *disabling* telemetry, by setting the
`%_WriteTelemetryProperties%` env var to `false`.

Author: jonathanpeppers

build-tools/automation/yaml-templates/variables.yaml

@@ -38,3 +38,6 @@ variables:
 # Workaround: https://github.com/dotnet/sdk/issues/26965
 - name: DOTNET_CLI_DO_NOT_USE_MSBUILD_SERVER
   value: true
+# Workaround: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1585820
+- name: _WriteTelemetryProperties
+  value: false