Change nuget build suffix for PRs to start with "-pr"

This is an attempt to fix the build, since this step of the build is failing in PRs.

appveyor.yml

@@ -64,7 +64,7 @@ after_build:
       if ($Env:APPVEYOR_REPO_TAG -eq "true") {
         nuget pack src/Discord.Net/Discord.Net.nuspec -OutputDirectory "artifacts" -properties suffix=""
       } else {
-        nuget pack src/Discord.Net/Discord.Net.nuspec -OutputDirectory "artifacts" -properties suffix="-$Env:BUILD"
+        nuget pack src/Discord.Net/Discord.Net.nuspec -OutputDirectory "artifacts" -properties suffix="-pr$Env:BUILD"
       }
     }
 - ps: if ($isWindows) { Get-ChildItem artifacts/*.nupkg | % { Push-AppveyorArtifact $_.FullName -FileName $_.Name } }