[vswhere] set WorkingDirectory

[jonathanpeppers]

Context: http://build.devdiv.io/2547192
Context: https://stackoverflow.com/questions/990562/win32exception-the-directory-name-is-invalid

We seem to have hit an odd failure during the MSBuild tests:

Unhandled Exception: System.ComponentModel.Win32Exception: The directory name is invalid
    at System.Diagnostics.Process.StartWithCreateProcess(ProcessStartInfo startInfo)
    at System.Diagnostics.Process.Start()
    at System.Diagnostics.Process.Start(ProcessStartInfo startInfo)
    at Xamarin.Android.Tools.VSWhere.MSBuildLocator.Exec(String fileName, String args) in E:\A\_work\1404\s\tools\vswhere\MSBuildLocator.cs:line 55
    at Xamarin.Android.Tools.VSWhere.MSBuildLocator.QueryLatest() in E:\A\_work\1404\s\tools\vswhere\MSBuildLocator.cs:line 28
    at Xamarin.Android.Build.XABuildPaths..ctor() in E:\A\_work\1404\s\tools\xabuild\XABuildPaths.cs:line 124
    at Xamarin.Android.Build.XABuild.Main() in E:\A\_work\1404\s\tools\xabuild\XABuild.cs:line 17

From looking up other examples of this error on the web, it seems that
a weird WorkingDirectory from the build environment would cause
this? WorkingDirectory was not set at all, so we would pickup
whatever the default is.

Let's just set ProcessStartInfo.WorkingDirectory to avoid a
potential problem here.

[jonathanpeppers]

Windows test run looks good, 1 test failure.

There does look like there is another random aapt2 failure:

E:\A\_work\1214\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Aapt2.targets(146,3): Value cannot be null.
Parameter name: path
   at System.IO.File.Delete(String path)
   at Xamarin.Android.Tasks.Aapt2Link.DoExecute()
   at System.Threading.Tasks.Task.InnerInvoke()
   at System.Threading.Tasks.Task.Execute() [E:\A\_work\1214\s\bin\TestRelease\temp\CompileBeforeUpgradingNuGet\UnnamedProject.csproj]

I will see if I can figure out what is wrong here--we should fix it.