[ci] 2 unit tests fails with date comparison on nightly-main

[sebsto]

See https://github.com/awslabs/swift-aws-lambda-events/actions/runs/18697673924/job/53319258143

✘ Test simpleEventFromJSON() recorded an issue at S3Tests.swift:114:9: Expectation failed: (record.eventTime → 2020-01-13 09:25:40 +0000) == (Date(timeIntervalSince1970: 1_578_907_540.621) → 2020-01-13 09:25:40 +0000)
✘ Test objectRemovedEvent() recorded an issue at S3Tests.swift:138:9: Expectation failed: (record.eventTime → 2020-01-13 09:25:40 +0000) == (Date(timeIntervalSince1970: 1_578_907_540.621) → 2020-01-13 09:25:40 +0000)

[sebsto]

Might be related to #85 ?
@t089 do you have time to have a look at this ? It's probably related to the implementation of @ISO8601WithFractionalSecondsCoding