Make all EnvVar values in sig-scalability-presets.yaml strings

[hbostan]

In the Preset stuct environment variables are stored in a slice of v1.EnvVar. Which, in turn assumes the value is a string. We should be consistent and make the values in the sig-scalability-presets.yaml strings too.

[k8s-ci-robot]

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: hbostan
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[krzyzacy]

/ok-to-test