Use api.googleads.serviceAccountSecretsPath

[hamsterready]

As per documentation https://developers.google.com/google-ads/api/docs/client-libs/java/config-file and code https://github.com/googleads/google-ads-java/blob/b994e0f125855cd8f9c3e63d732ceefaae27cd8c/google-ads/src/main/java/com/google/ads/googleads/lib/GoogleAdsClient.java#L648

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[hamsterready]

@googlebot I signed it!