* Update to Mapbox-iOS-SDK 6.3.0

The 6.0.0 upgrade had no breaking changes:

> This major release does not include any breaking changes to public
> APIs. We are treating this release as a SEMVER major change because
> our installation instructions have changed.
https://github.com/mapbox/mapbox-gl-native-ios/releases/tag/ios-v6.0.0

I did however need to cast the `accessToken` as a `String` to satisfy
the compiler after this upgrade.

Prior to the upgrade I was seeing crashes from this bug, which was fixed
in 6.2: https://github.com/mapbox/mapbox-gl-native-ios/issues/485

So far I don't see any issues at all after the upgrade.

Developers will need to configure their access token in order to fetch
the mapbox sdk for this an future versions, described here:
https://docs.mapbox.com/ios/maps/guides/install/

* Setup .netrc for iOS mapbox sdk download

Writes a `~/.netrc` file containing the credentials needed to download
the iOS mapbox sdk. I'm not experienced with github workflow so I'm just
hoping that I can write to the home directory of whatever container it
runs in. This should allow curl to find and use the given credentials.

Co-authored-by: tobrun <[email protected]>