`BUILDKITE_PULL_REQUEST_BASE_BRANCH` not updated when PR base branch changed

It seems like https://github.com/buildkite/feedback/issues/306 is back.

I don't know how to provide more information, as this is a private repo, but:

- I created a PR on a branch with the base branch `foo`
- I updated the PR to be based on `master`
- A new build was triggered (automatically, but this also happened when I tried to rebuild manually)
- in the Environment tab, I see `BUILDKITE_PULL_REQUEST_BASE_BRANCH="foo"`

This is causing our builds to fail.

The report names no repository files, tests, or entry points. Start by reproducing the pull-request base-branch change and inspect how the agent populates BUILDKITE_PULL_REQUEST_BASE_BRANCH during a new or manually rebuilt build. Done means the variable reflects the updated base branch and the behavior is covered by a regression test.

Written by the indexing model from the issue text.