Allow override of build tool framework version (#2283)

* Allow override of build tool framework version

* Update env.var check

Author: NikolaMilosavljevic

eng/common/tools.sh

@@ -341,7 +341,12 @@ function InitializeBuildTool {
   # return values
   _InitializeBuildTool="$_InitializeDotNetCli/dotnet"
   _InitializeBuildToolCommand="msbuild"
-  _InitializeBuildToolFramework="net8.0"
+  # use override if it exists - commonly set by source-build
+  if [[ "${_OverrideArcadeInitializeBuildToolFramework:-x}" == "x" ]]; then
+    _InitializeBuildToolFramework="net8.0"
+  else
+    _InitializeBuildToolFramework="${_OverrideArcadeInitializeBuildToolFramework}"
+  fi
 }
 
 # Set RestoreNoCache as a workaround for https://github.com/NuGet/Home/issues/3116