restore discovery logic

Author: SteveL-MSFT

dsc_lib/src/discovery/command_discovery.rs

@@ -126,7 +126,7 @@ impl CommandDiscovery {
         let mut paths: Vec<PathBuf> = vec![];
 
         let dsc_resource_path = env::var_os("DSC_RESOURCE_PATH");
-        if resource_path_setting.allow_env_override {
+        if resource_path_setting.allow_env_override && dsc_resource_path.is_some() {
             if let Some(value) = dsc_resource_path {
                 debug!("DSC_RESOURCE_PATH: {:?}", value.to_string_lossy());
                 using_custom_path = true;