Enable PowerShell Core support in Kubernetes Executor

What does this MR do?

It makes the necessary changes for the Kubernetes executor and Powershell shell to support the pwsh shell value and use the correct helper image.

Why was this MR needed?

See #13145 (closed)

What's the best way to test this MR?

1. Start a Kubernetes dev cluster locally with minikube (minikube start)

2. Switch the Docker client to the Kubernetes cluster:

   eval $(minikube docker-env)

3. Build the helper image and Runner so it is available in minikube's Docker daemon image cache:

   make helper-dockerarchive-host runner-bin-host

4. Register a Runner to run builds on the local cluster (define $YOUR_TOKEN):

   gitlab-runner register --executor "kubernetes" --kubernetes-image mcr.microsoft.com/powershell --url "https://gitlab.com/" --description "kubernetes-runner" --tag-list "kubernetes,gitlab.com,pwsh" --locked="false" --access-level="not_protected" --registration-token="<your token>" --non-interactive

5. Start the runner:

   out/binaries/gitlab-runner run

6. Create a pipeline with the following .gitlab-ci.yml:

   start_evaluation:
     image: mcr.microsoft.com/powershell
     script:
       - Get-Host
     tags: [kubernetes, pwsh]

7. The pipeline should pass, as seen in this job

   Job log

   Running with gitlab-runner 13.9.0~beta.65.g4262cecd (4262cecd)
     on Kubernetes executor zqMtkFxL
   Resolving secrets
   00:00
   Preparing the "kubernetes" executor
   00:00
   WARNING: Namespace is empty, therefore assuming 'default'.
   Using Kubernetes namespace: default
   WARNING: Pulling GitLab Runner helper image from Docker Hub. Helper image is migrating to registry.gitlab.com, for more information see https://docs.gitlab.com/runner/configuration/advanced-configuration.html#migrating-helper-image-to-registrygitlabcom
   Using Kubernetes executor with image mcr.microsoft.com/powershell ...
   Preparing environment
   00:04
   Waiting for pod default/runner-zqmtkfxl-project-16892582-concurrent-0sfltt to be running, status is Pending
   Running on runner-zqmtkfxl-project-16892582-concurrent-0sfltt via 
   pedros-macbookpro16.home.pombei.ro...
   Getting source from Git repository
   00:03
   Fetching changes with git depth set to 50...
   Initialized empty Git repository in /builds/pedropombeiro/playground/.git/
   Created fresh repository.
   Checking out cb4bb3d5 as k8s-pwsh-test...
   git-lfs/2.11.0 (GitHub; linux amd64; go 1.13.11; git fceccfb7bd)
   Skipping Git submodules setup
   Executing "step_script" stage of the job script
   00:01
   $ Get-Host
   Name             : ConsoleHost
   Version          : 7.1.1
   InstanceId       : 20d09587-e70f-4fb7-9164-1eeafbb3aac2
   UI               : System.Management.Automation.Internal.Host.InternalHostUserI
                      nterface
   CurrentCulture   : en-US
   CurrentUICulture : en-US
   PrivateData      : Microsoft.PowerShell.ConsoleHost+ConsoleColorProxy
   DebuggerEnabled  : True
   IsRunspacePushed : False
   Runspace         : System.Management.Automation.Runspaces.LocalRunspace
   Cleaning up file based variables
   00:00
   Job succeeded

What are the relevant issue numbers?

Closes #13145 (closed)