AKS Get-Credentials doesn't handle KUBECONFIG with multiple files

**Describe the bug**

When a user's `KUBECONFIG` environment variable contains more than one file, then `az aks get-credentials...` mistakenly treats the value as a single path.

As per [the official documentation](https://kubernetes.io/docs/concepts/configuration/organize-cluster-access-kubeconfig/#the-kubeconfig-environment-variable) this colon-delimited format is explicitly supported on Mac and Linux.

This behaviour appears to have been introduced by https://github.com/Azure/azure-cli/pull/18704 and is present in v2.26.0 and above.

e.g.

If my `KUBECONFIG` looks like so:

`KUBECONFIG=/home/me/.kube/config:/home/me/.kube/anotherconfig:/home/me/.kube/yetanotherconfig`

Then when I run `az aks get-credentials...` then it will attempt to write the file to a file at the path `/home/me/.kube/config:/home/me/.kube/anotherconfig:/home/me/.kube/yetanotherconfig` rather than one of the single files contained within.

**To Reproduce**

```sh
$ KUBECONFIG=/home/me/.kube/config:/home/me/.kube/anotherconfig
$ az aks get-credentials ...
$ cat "/home/me/.kube/config:/home/me/.kube/anotherconfig"
```

**Expected behavior**

`az aks get-credentials...` should write AKS credentials to _one_ of the files contained within `KUBECONFIG` only, but not sure how it should choose which to use.

**Environment summary**

Install method: homebrew
Mac Version: 11.4

```
$ az --version
azure-cli 2.26.0

core 2.26.0
telemetry 1.0.6

Python location '/usr/local/Cellar/azure-cli/2.26.0/libexec/bin/python'
Extensions directory '/Users/paddy/.azure/cliextensions'

Python (Darwin) 3.8.11 (default, Jun 29 2021, 03:08:07)
[Clang 12.0.5 (clang-1205.0.22.9)]
```

**Additional Context**

N/A