Fix exec provider

[tomasaschan]

What type of PR is this?

/kind bug
/kind regression

What this PR does / why we need it:

The ExecProvider has a regression, probably due to 8c60fe9, resulting in broken auth when using the exec provider in provided kubeconfig.

Which issue(s) this PR fixes:

Fixes #2334

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Fixes bug that would fail authentication when using the exec-provider with a specific cluster selected

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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This pull-request has been approved by: tomasaschan
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[tomasaschan]

This potentially also fixes #2333, #2338 and #2339; I don't have access to the other third-party platforms available to test that out, but it seems likely that those auth errors are symptoms of the same thing.

[yliaog]

/assign