CSHARP-5332: Fix EFCore test script (#1507)

Author: sanych-sun

evergreen/run-external-script.sh

@@ -14,7 +14,10 @@ git clone -b "${GIT_BRANCH:-main}" --single-branch "${GIT_REPO}" .
 # add/adjust nuget.config pointing to myget so intermediate versions could be restored
 if [ -f "./nuget.config" ]; then
   echo "Adding myget into nuget.config"
-  dotnet nuget add source https://www.myget.org/F/mongodb/api/v3/index.json -n myget.org --configfile ./nuget.config
+  NUGET_SOURCES=$(dotnet nuget list source --format short)
+  if [[ ${NUGET_SOURCES} != *"https://www.myget.org/F/mongodb/api/v3/index.json"* ]];then
+      dotnet nuget add source https://www.myget.org/F/mongodb/api/v3/index.json -n myget.org --configfile ./nuget.config
+  fi
 else
   echo "Creating custom nuget.config"
   cat > "nuget.config" << EOL