[build] Override default yaml checkout behavior

[pjcollins]

We're hitting a couple of git related issues on certain machines, most
commonly around submodule updating on Windows. Rather than relying on an
msbuild setup task to do this, we should override the default pipeline
source checkout behavior. We'll now fetch a clean tree and recursively
clone all relevant submodules for each build stage.

[jonathanpeppers]

Yeah I had these settings set on the old “Xamarin.Android (Windows)” job.

[jonathanpeppers]

Well, we got past the git checkout, but there is new breakage from 055dabe:

"E:\A\_work\2218\s\tests\Runtime-MultiDex\Mono.Android-TestsMultiDex.csproj" (default target) (38) ->
(_ConvertPdbFiles target) -> 
  E:\A\_work\2218\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Common.targets(2060,2): warning : Could not load file or assembly 'Mono.CompilerServices.SymbolWriter, Version=4.0.0.0, Culture=neutral, PublicKeyToken=0738eb9f132ed756' or one of its dependencies. The system cannot find the file specified. [E:\A\_work\2218\s\tests\Runtime-MultiDex\Mono.Android-TestsMultiDex.csproj]
E:\A\_work\2218\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Common.targets(2060,2): warning :    at Pdb2Mdb.Converter.Convert(String filename) [E:\A\_work\2218\s\tests\Runtime-MultiDex\Mono.Android-TestsMultiDex.csproj]
E:\A\_work\2218\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Common.targets(2060,2): warning :    at Xamarin.Android.Tasks.ConvertDebuggingFiles.Execute() [E:\A\_work\2218\s\tests\Runtime-MultiDex\Mono.Android-TestsMultiDex.csproj]

[pjcollins]

Yeah, I think we should try to get this and #3158 in asap to help stabilize things.