Xamarin.Android build Task + $(SolutionDir)

I was recently setting up build automation for a Xamarin.Forms project I'm working on and found that the initial template was not able to build my project. I was getting an error like:

C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets(1987,5):
warning MSB3245: Could not resolve this reference. Could not locate the assembly
"Newtonsoft.Json, Version=9.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed, processorArchitecture=MSIL".
Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors.
For SearchPath "{HintPathFromItem}".
Considered "*Undefined*packages\Newtonsoft.Json.9.0.1\lib\portable-net45+wp80+win8+wpa81\Newtonsoft.Json.dll", but it didn't exist.

Notice the *Undefined* in the considered path.

After a little digging, here's what I found.

In all the other projects in the solution, I include packages using the $(SolutionDir) variable in the HintPath like:

<Reference Include="Ninject, Version=3.0.0.0, Culture=neutral, PublicKeyToken=c7192dc5380945e7, processorArchitecture=MSIL">
  <HintPath>$(SolutionDir)packages\Portable.Ninject.3.3.1\lib\portable-net4+sl5+wp8+win8+wpa81+monotouch+monoandroid+Xamarin.iOS\Ninject.dll</HintPat>
  <Private>False</Private>
</Reference>

Because the Xamarin.Android task is building a project, there is no $(SolutionDir) available. If you use the VisualStudio build to build the Xamarin.Forms solution, it will build find. So how to get Xamain.Android to replace the $(SolutionDir) variable?

Add a Build Variable!

To figure out the value of the build variable, at the top of the logs for the Xamarin.Android build step you'll find a few lines like this:

Build started 9/18/2017 8:59:48 PM.
Project "d:\a\3\s\DEV\App.Mobile.Droid\App.Mobile.Droid.csproj" on node 1 (PackageForAndroid target(s)).

In my case, the directory was d:\a\3\s\DEV\ because in the Get Sources step, I specified DEV as the folder to put the sources into. If you don't do this, it will typically be something like c:\a\1\s\
At any rate, after adding a build variable with this value, the Xamarin.Android task is able to properly find the packages required to build the solution.