[Bug] az pipelines variable delete does not work with —variable-name

**Describe the bug**
Running:

```sh
$: az pipelines variable delete —pipeline-name “foo” —name “var_name” # rest of parameters
```

does not work. However, running:

```sh
$: az pipelines variable delete —pipeline-id $id —name “var_name” # rest of parameters
```

does. This works as intended when one runs `az pipelines variable create`.

`—pipeline-name` seems to be a supported parameter when running `az pipelines variable delete —help`.

Upon looking at `—debug`, it seems that the client tries to do this:

```
DEBUG: urllib3.connectionpool: https://dev.azure.com:443 “GET /org_name/project_name/_apis/build/Definitions?name=foo
```

instead of this

```
DEBUG: urllib3.connectionpool: https://dev.azure.com:443 “GET /org_name/project_name/_apis/build/Definitions/$id?name=foo
```

**To Reproduce**
Azure Cli Version: 2.8.0

Azure-Devops extension version: 0.18.0

Steps to reproduce the behavior:

1. Create a pipeline; doesn’t matter how.
2. Create a variable: `AZURE_DEVOPS_EXT_PAT=pat az pipelines variable create —name foo —project project —org https://dev.azure.com/org —pipeline-name pipeline —value bar`
3. Delete that variable: `AZURE_DEVOPS_EXT_PAT=pat az pipelines variable delete —name foo —project project —org https://dev.azure.com/org —pipeline-name pipeline`

**Expected behavior**

The variable should get deleted.