Alternative `privateKeySecretRef` name won't work

[abrondijk]

If I give the secret that holds the transip private key a name other than transip-secret, the created challenge will fail on line:

cert-manager-webhook-transip/transip/solver.go

Line 192 in cb927be

return nil, fmt.Errorf("failed to load secret '%s/%s'", ns, name)

Error presenting challenge: get credential error: failed to load secret `namespace/secret-name`

The challenge can also not be deleted, with the same error.