Unquoted KUBECONFIG variable in run_tests.sh

## Description

The `KUBECONFIG` variable is used without quotes, which will break if the path contains spaces.

## Location

https://github.com/validatedpatterns/multicloud-gitops/blob/main/tests/interop/run_tests.sh

```bash
--kubeconfig $KUBECONFIG
```

## Expected Behavior

Quote the variable: `--kubeconfig "$KUBECONFIG"`