Error message when Terraform CLI cannot be loaded from sources does not list sources

### terraform-plugin-testing version

```
v1.2.0
```

### Expected Behavior

When the Terraform CLI cannot be found, an error message like

```
cannot run Terraform provider tests: failed to find or install Terraform CLI from [ ]: ...
```

should be returned.

### Actual Behavior

The error message was

```
cannot run Terraform provider tests: failed to find or install Terraform CLI from [0xc0018cba40 0xc005f65180]: ...
```

### Steps to Reproduce

Fail to locate the Terraform CLI when running tests, e.g. a network failure