there is no appveyor-retry on Linux

Author: bodewig

appveyor.yml

@@ -45,7 +45,7 @@ for:
     cache:
       - packages -> **/packages.config
     before_build:
-      - appveyor-retry nuget restore -DisableParallelProcessing -verbosity detailed XMLUnit.NET.NetFramework.sln
+      - nuget restore -DisableParallelProcessing -verbosity detailed XMLUnit.NET.NetFramework.sln
     build_script:
       - xbuild XMLUnit.NET.NetFramework.sln