Mapbox-gl-native: Nightly build workflow lacks s3 upload permissions

[friedbunny]

The nightly iOS build added in #8337 fails to upload the result to s3:

* Uploading mapbox-ios-sdk-nightly-dynamic-2017-03-10.zip to s3…
upload failed: ../mapbox-ios-sdk-nightly-dynamic-2017-03-10.zip to s3://mapbox/mapbox-gl-native/ios/builds/mapbox-ios-sdk-nightly-dynamic-2017-03-10.zip
An error occurred (AccessDenied) when calling the CreateMultipartUpload operation: Access Denied

This workflow appears to be missing the necessary permission to upload to s3://mapbox/mapbox-gl-native/ios/builds/, though it _can_ upload to s3://mapbox/mapbox-gl-native/metrics/binary-size/.

/cc @kkaefer

[1ec5]

By the way, this is causing the Bitrise badge on the homepage to go red. We may want to disable the scheduled build on Bitrise until we have time to look into this failure, since the failure is specific to the nightly build and not representative of the project as a whole.

[friedbunny]

Given that the nightly workflow is still generating valuable build size metrics and the fact that the badge doesn’t represent the state of our current working branch anyway, I’d vote to let it stay red for now. I’ll aim to fix this s3 permissions issue today.

[kkaefer]

The current Cloudformation template only allows uploading to /mapbox-gl-native/metrics/. You'll need to add the path here and run the Cloudformation deployment process.