Fix iOS sourcemaps upload

[louiszawadzki]

What does this PR do?

Make iOS sourcemaps upload more reliable. We set the path for iOS sourcemaps to an absolute path, so that we don't have issues of being in different directories when outputting the sourcemaps and when getting it for the upload. This change had already been made in the wizard, but wasn't replicated here.

Motivation

Fix https://github.com/DataDog/expo-datadog/issues/13

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests
• [ ] Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• [ ] Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)
• [ ] If this PR is auto-generated, please make sure also to manually update the code related to the change

[louiszawadzki]

I made an EAS build that passes thanks to this change