🌱 clusterctl: reset global viper state in viperReader Set test

[Cozm1n1412]

What this PR does / why we need it:
viperReader uses the global viper instance. Test_viperReader_Set calls Set("FOO", "bar"), and since viper.Set overrides take precedence over env vars, the value leaks into Test_viperReader_Get on the next -count iteration (Read from env gets bar instead of foo). go test -count=2 ./cmd/clusterctl/client/config/ fails on main.

This resets the global viper state via t.Cleanup(viper.Reset). Test-only change. A longer-term option would be switching viperReader to its own viper.New() instance; happy to do that instead if preferred.

Verified: go test -count=3 ./cmd/clusterctl/client/config/ passes.

/kind cleanup
/area clusterctl

[kubernetes-prow]

Hi @Cozm1n1412. Thanks for your PR.

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[kubernetes-prow]

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