Implement replacement for appveyor-retry

appveyor.yml

@@ -26,7 +26,23 @@ init:
 build_script:
 - ps: >-
     if ($isLinux) {
-      dotnet restore Discord.Net.sln -v Minimal /p:BuildNumber="$Env:BUILD" /p:IsTagBuild="$Env:APPVEYOR_REPO_TAG"
+      # AppVeyor Linux images do not have appveyor-retry, which retries the commands a few times
+      # until the command exits with code 0.
+      # So, this is done with a short script.
+      $code = 0
+      $counter = 0
+      do {
+        dotnet restore Discord.Net.sln -v Minimal /p:BuildNumber="$Env:BUILD" /p:IsTagBuild="$Env:APPVEYOR_REPO_TAG"
+        $code = $LASTEXITCODE
+        $counter++
+
+        if ($code -ne 0)
+        {
+          # Wait 5s before attempting to run again
+          Start-sleep -Seconds 5
+        }
+
+      } until ($counter -eq 5 -or $code -eq 0)
     } else {
       appveyor-retry dotnet restore Discord.Net.sln -v Minimal /p:BuildNumber="$Env:BUILD" /p:IsTagBuild="$Env:APPVEYOR_REPO_TAG"
     }