Rename SourceBuildUseMonoRuntime property, which is not SB specific

This is a coordinated cross-repo change, and might break the build until
all four PRs are merged. The other three are:

Ref: dotnet/source-build#4165

Author: Jo Shields

eng/DotNetBuild.props

@@ -64,7 +64,7 @@
           BeforeTargets="GetSourceBuildCommandConfiguration">
 
     <PropertyGroup>
-      <InnerBuildArgs Condition="'$(SourceBuildUseMonoRuntime)' == 'true'">$(InnerBuildArgs) /p:CrossgenOutput=false</InnerBuildArgs>
+      <InnerBuildArgs Condition="'$(DotNetBuildUseMonoRuntime)' == 'true'">$(InnerBuildArgs) /p:CrossgenOutput=false</InnerBuildArgs>
     </PropertyGroup>
   </Target>