Allow customizing setup and create commands in integration tests

> Looks like we both hit the same issue 😄 (I fixed it doing this: [cd2cf5a](https://github.com/coder/terraform-provider-coder/commit/cd2cf5a8f7b675c46b4a3b423c1936cfee78b665))
>
> I think I prefer your solution though 👍 (but not entirely sure why CODER_TASK_ID is being passed through as a `uuid.Nil`)

I think it's because we're creating a workspace and not a task in the integration test 🤦

```
_, rc = execContainer(ctx, t, ctrID, fmt.Sprintf(`coder create %s -t %s %s --yes`, tt.name, tt.name, includeParameters))
```

_Originally posted by @johnstcn in https://github.com/coder/terraform-provider-coder/issues/456#issuecomment-3463934169_

It would be useful to be able to customize the commands used to set up the template and create the resource in the integration tests. These are currently hard-coded.