Set telemetry output in portable build.sh

Seems this was missed who knows how long ago, so add it just in case.

FYI This only affects our builds, not our users' privacy.

portable/build.sh

@@ -28,6 +28,7 @@ case ${BUILD_TYPE} in
         APPHOST="-p:UseAppHost=true"
     ;;
 esac
+export DOTNET_CLI_TELEMETRY_OPTOUT=1
 dotnet publish Jellyfin.Server --configuration Release ${RUNTIME} --output ${BUILD_DIR}/jellyfin/ -p:DebugSymbols=false -p:DebugType=none ${APPHOST}
 popd