build.cake: make sure to use the last MSBuild version

Somehow, cake was defaulting to use the old MSBuild (whole path
being `C:/windows/Microsoft.NET/Framework/v4.0.30319/MSBuild.exe`)
instead of the one in the VS2019 folder (which should be in
`%ProgramFiles(x86)%\Microsoft Visual Studio\2019\Enterprise\MSBuild\Current\Bin\MSBuild.exe`
but is better to be located via vswhere).

This fixes many build problems happening in the GitHubActions VM:

```
"D:\a\ZXing.Net.Xamarin\ZXing.Net.Xamarin\ZXing.Net.Mobile.sln" (Build target) (1) ->
"D:\a\ZXing.Net.Xamarin\ZXing.Net.Xamarin\Source\ZXing.Net.Mobile.Android\ZXing.Net.Mobile.Android.csproj" (default target) (5) ->
  D:\a\ZXing.Net.Xamarin\ZXing.Net.Xamarin\Source\ZXing.Net.Mobile.Android\ZXing.Net.Mobile.Android.csproj(82,60): error MSB4066: The attribute "Version" in element <PackageReference> is unrecognized.

"D:\a\ZXing.Net.Xamarin\ZXing.Net.Xamarin\ZXing.Net.Mobile.sln" (Build target) (1) ->
"D:\a\ZXing.Net.Xamarin\ZXing.Net.Xamarin\Source\ZXing.Net.Mobile.iOS\ZXing.Net.Mobile.iOS.csproj" (default target) (6) ->
  D:\a\ZXing.Net.Xamarin\ZXing.Net.Xamarin\Source\ZXing.Net.Mobile.iOS\ZXing.Net.Mobile.iOS.csproj(72,3): error MSB4019: The imported project "C:\Program Files (x86)\MSBuild\Xamarin\iOS\Xamarin.iOS.CSharp.targets" was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.

"D:\a\ZXing.Net.Xamarin\ZXing.Net.Xamarin\ZXing.Net.Mobile.sln" (Build target) (1) ->
"D:\a\ZXing.Net.Xamarin\ZXing.Net.Xamarin\Source\ZXing.Net.Mobile.WindowsUniversal\ZXing.Net.Mobile.WindowsUniversal.csproj" (default target) (7) ->
  D:\a\ZXing.Net.Xamarin\ZXing.Net.Xamarin\Source\ZXing.Net.Mobile.WindowsUniversal\ZXing.Net.Mobile.WindowsUniversal.csproj(155,3): error MSB4019: The imported project "C:\Program Files (x86)\MSBuild\Microsoft\WindowsXaml\v11.0\Microsoft.Windows.UI.Xaml.CSharp.targets" was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.
```

Solution taken from https://stackoverflow.com/a/45251462/544947
Hint taken from https://stackoverflow.com/a/55269143/544947

build.cake

@@ -1,4 +1,5 @@
 #tool nuget:?package=XamarinComponent
+#tool nuget:?package=vswhere
 
 #addin nuget:?package=Cake.Android.SdkManager
 #addin nuget:?package=Cake.XCode
@@ -41,9 +42,20 @@ Task("libs")
 	NuGetRestore("./ZXing.Net.Mobile.sln");
 	NuGetRestore("./ZXing.Net.Mobile.Forms.sln");
 
+	DirectoryPath vsLatest = VSWhereLatest();
+	string msbuildExeRelativePath = "./MSBuild/Current/Bin/MSBuild.exe";
+	FilePath msbuildPathFromVS = (vsLatest == null) ? null : vsLatest.CombineWithFilePath(msbuildExeRelativePath); 
+
 	var config = IsRunningOnWindows() ? "ReleaseWin" : "ReleaseMac";
-	MSBuild ("./ZXing.Net.Mobile.sln", c => c.SetConfiguration(config).SetMSBuildPlatform(MSBuildPlatform.x86));
-	MSBuild ("./ZXing.Net.Mobile.Forms.sln", c => c.SetConfiguration(config).SetMSBuildPlatform(MSBuildPlatform.x86));
+
+	var msbuildSettings = new MSBuildSettings {
+		Configuration = config,
+		MSBuildPlatform = MSBuildPlatform.x86,
+		ToolPath = msbuildPathFromVS,
+	};
+
+	MSBuild ("./ZXing.Net.Mobile.sln", msbuildSettings);
+	MSBuild ("./ZXing.Net.Mobile.Forms.sln", msbuildSettings);
 });
 
 Task ("samples")