Backend: adjust legacy obj path (#218)

Using the same folder for two projects (e.g.
GWallet.Backend.fsproj and GWallet.Backend-legacy.fsproj) that
use different frameworks (e.g. NetStandard2.0 one and .NET4.x
one), causes problems due to incompatible caches in obj folder,
so by adjusting the cache obj folder for the legacy version we
workaround this problem.

src/GWallet.Backend/GWallet.Backend-legacy.fsproj

@@ -18,6 +18,7 @@
         /warnon:3218
         /warnon:0193
     </OtherFlags>
+    <BaseIntermediateOutputPath>obj\legacy\</BaseIntermediateOutputPath>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
     <DebugSymbols>true</DebugSymbols>