[build] delete platform-31 folder on test jobs

[jonathanpeppers]

Context: #6089

We currently have lots of test failures due to the warning:

warning XA1008: The TargetFrameworkVersion (Android API level 30) is lower than the targetSdkVersion (31). Please increase the `$(TargetFrameworkVersion)` or decrease the `android:targetSdkVersion` in the `AndroidManifest.xml` so that the API levels match.

Looking at Properties\AndroidManifest.xml, Xamarin.ProjectTools
generated:

<uses-sdk android:minSdkVersion="19" android:targetSdkVersion="31" />

This appears to be due to the GetMaxInstalledPlatform() method:

https://github.com/xamarin/xamarin-android/blob/1b71da0cd3a44861b006ac41c77a24eedf066281/src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/AndroidSdkResolver.cs#L121-L138

I think what happened is:

• [Mono.Android] Bind API-S Beta 3 and enumify #6089 is opened, platform-31 gets installed on many of the CI
  machines.
• Future PRs install platform-31 side-by-side platform-S.

And so we ended up with test failures...

To fix this for now, manually delete the sdk/platform-31 folder on
test jobs.

[jonathanpeppers]

This seems to have worked, I see it deleting files:

Generating script.
Script contents:
rm -rfv "$HOME/Library/Android/sdk/platforms/android-31"
========================== Starting Command Output ===========================
/bin/bash --noprofile --norc /Users/runner/work/_temp/7b27620b-100e-4ba6-b5ed-b5c85b56e8aa.sh
/Users/runner/Library/Android/sdk/platforms/android-31/core-for-system-modules.jar
/Users/runner/Library/Android/sdk/platforms/android-31/package.xml
/Users/runner/Library/Android/sdk/platforms/android-31/android.jar
/Users/runner/Library/Android/sdk/platforms/android-31/skins/WSVGA/background_port.png
...

https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=4996973&view=logs&j=28e101f7-a5d4-5072-d882-1de9b1157eea&t=4b39297b-abed-5a38-7214-e13de457d588&l=14

There are other failures like:

  Restored C:\a\_work\1\s\build-tools\xaprepare\xaprepare\package-download.proj (in 13.72 sec).
stderr | System.ArgumentException: An item with the same key has already been added. Key: Microsoft.NET.Runtime.Emscripten.Node
stderr |    at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior) in System.Private.CoreLib.dll:token 0x6006799+0x1a0
stderr |    at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value) in System.Private.CoreLib.dll:token 0x600678c+0x0
stderr |    at Microsoft.NET.Sdk.WorkloadManifestReader.WorkloadResolver.RefreshWorkloadManifests() in Microsoft.DotNet.TemplateLocator.dll:token 0x6000070+0x1fd
stderr |    at Microsoft.NET.Sdk.WorkloadManifestReader.WorkloadResolver..ctor(IWorkloadManifestProvider manifestProvider, String[] dotnetRootPaths, String[] currentRuntimeIdentifiers) in Microsoft.DotNet.TemplateLocator.dll:token 0x600006f+0x31
stderr |    at Microsoft.NET.Sdk.WorkloadManifestReader.WorkloadResolver.Create(IWorkloadManifestProvider manifestProvider, String dotnetRootPath, String sdkVersion) in Microsoft.DotNet.TemplateLocator.dll:token 0x600006c+0x8a
stderr |    at Microsoft.DotNet.Workloads.Workload.Install.WorkloadInstallCommand..ctor(ParseResult parseResult, IReporter reporter, IWorkloadResolver workloadResolver, IInstaller workloadInstaller, INuGetPackageDownloader nugetPackageDownloader, IWorkloadManifestUpdater workloadManifestUpdater, String dotnetDir, String userHome, String tempDirPath, String version) in dotnet.dll:token 0x6000207+0x17f
stderr |    at Microsoft.DotNet.Workloads.Workload.WorkloadCommand.<get_SubCommands>b__9_0(ParseResult appliedOption) in dotnet.dll:token 0x6000152+0x0
stderr |    at Microsoft.DotNet.Cli.DotNetTopLevelCommandBase.RunCommand(String[] args) in dotnet.dll:token 0x6000938+0x44
stderr |    at Microsoft.DotNet.Workloads.Workload.WorkloadCommand.Run(String[] args) in dotnet.dll:token 0x6000150+0x6
stderr |    at Microsoft.DotNet.Cli.Program.ProcessArgs(String[] args, TimeSpan startupTime, ITelemetry telemetryClient) in dotnet.dll:token 0x60009a1+0x2e8
stderr |    at Microsoft.DotNet.Cli.Program.Main(String[] args) in dotnet.dll:token 0x600099f+0x6f
  Error: dotnet workload install failed.

I think we need #6098 for these.