Do not reuse existing installation of .NET CLI in SB prep script (#17892)

Author: mthalman

src/SourceBuild/content/prep.sh

@@ -156,6 +156,7 @@ function BootstrapArtifacts {
 # Check for the version of dotnet to install
 if [ "$installDotnet" == true ]; then
   echo "  Installing dotnet..."
+  use_installed_dotnet_cli=false
   (source ./eng/common/tools.sh && InitializeDotNetCli true)
 fi