Potential race condition when getting PR labels for releasing

**Describe the bug**

[The `label` function](https://github.com/intuit/auto/blob/c6961698ab27ac61b1a0f44a9e20785d5ae11e88/packages/core/src/auto.ts#L899) by default gets the labels from the most recently merged PR on any branch, not the most recently merged PR on the target branch. This can cause a potential race condition messing up releases.

For example:

1. PR A is merged into `master`, awaiting a release to be generated.
2. PR B is merged into `next` immediately after, with a `skip-release` label.
3. When `auto` is run for PR A, it picks up the `skip-release` label from PR B, thus not end up creating a release.

This issue will happen for any project with multiple release tracks.

**To Reproduce**

Repeat the above steps, and observe that the release labels for PR B will be used for the release of PR A.

**Expected behavior**

When `auto` is run on a specific branch to create releases, it should only pick up PRs merged from that branch, and not any other branches.