Check if appveyor provides appveyor-retry by trying to run it

appveyor.yml

@@ -24,7 +24,15 @@ init:
 - ps: $Env:BUILD = "$($Env:APPVEYOR_BUILD_NUMBER.PadLeft(5, "0"))"
 
 build_script:
-- ps: appveyor-retry dotnet restore Discord.Net.sln -v Minimal /p:BuildNumber="$Env:BUILD" /p:IsTagBuild="$Env:APPVEYOR_REPO_TAG"
+- ps: |
+  if ($isLinux)
+  {
+    ./appveyor-retry dotnet restore Discord.Net.sln -v Minimal /p:BuildNumber="$Env:BUILD" /p:IsTagBuild="$Env:APPVEYOR_REPO_TAG"
+  }
+  else
+  {
+    appveyor-retry dotnet restore Discord.Net.sln -v Minimal /p:BuildNumber="$Env:BUILD" /p:IsTagBuild="$Env:APPVEYOR_REPO_TAG"
+  }
 - ps: dotnet build Discord.Net.sln -c "Release" /p:BuildNumber="$Env:BUILD" /p:IsTagBuild="$Env:APPVEYOR_REPO_TAG"
 after_build:
 - ps: dotnet pack "src\Discord.Net.Core\Discord.Net.Core.csproj" -c "Release" -o "../../artifacts" --no-build /p:BuildNumber="$Env:BUILD" /p:IsTagBuild="$Env:APPVEYOR_REPO_TAG"