Allow override of build tool framework version (#14181)

Author: NikolaMilosavljevic

Documentation/ArcadeSdk.md

@@ -1043,6 +1043,9 @@ If set to `true` the GetResourceString method is not included in the generated c
 #### `FlagNetStandard1XDependencies` (bool)
 If set to `true` the `FlagNetStandard1xDependencies` target validates that the dependency graph doesn't contain any netstandard1.x packages.
 
+#### `_OverrideArcadeInitializeBuildToolFramework` (string)
+If this environment variable is set, the value will be used to override the default Build Tools Framework version.
+
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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