kokoro: fix API key path
kokoro: fix API key path

Change-Id: I4fe8a5f6ccae1d3c8510be5d508d968a19fe8af1
Reviewed-on: https://code-review.googlesource.com/c/34330
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go