Update lint CI job to checkout PR merge ref

[mattt]

CI has been failing on recent PRs, like in this run.

Sources/Models/LanguageModel.swift:132:20: warning: [DontRepeatTypeInStaticProperties] remove the suffix 'Keys' from the name of the variable 'presentKeys'

However, this linting failure can't be resolved by any changes in the PR branch, including fixing the issue and disabling the check outright.

I noticed in the logs for action/checkout, that it's fetching origin/main, which is currently a2e184d.

Fetching the repository
  /opt/homebrew/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +a2e184dddb4757bc943e77fbe99ac6786c53f0b2:refs/remotes/origin/main
  From https://github.com/huggingface/swift-transformers
   * [new ref]         a2e184dddb4757bc943e77fbe99ac6786c53f0b2 -> origin/main

This PR updates the lint job to use the same checkout pattern as the shared workflow for testing.

[mattt]

CI continues to use the workflow file in main which checks out HEAD of main, so there's no way to get lint job to pass. We'll just have to YOLO merge and hope for the best 🤞