Add explanation on how to select runner manager node with nodeSelector

What does this MR do?

Recently we got two customer tickets who were trying to setup kubernetes runner that could schedule build pods on Windows nodes. They were getting the following errors:

  Warning  FailedMount  4s    kubelet            MountVolume.SetUp failed for volume "kube-api-access-24gfv" : chown c:\var\lib\kubelet\pods\fb506dd4-0a0a-4670-8492-1ab64172d5ac\volumes\kubernetes.io~projected\kube-api-access-24gfv\..2022_07_08_13_48_59.148971519\token: not supported by windows

After recearching this issue, we determined that we need to instruct the runner to select linux nodes for runner manager pod:

nodeSelector:
  kubernetes.io/os: linux

This Merge Request documents the error and the solution.

Customer tickets:

• https://gitlab.zendesk.com/agent/tickets/305078
• https://gitlab.zendesk.com/agent/tickets/302187

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