PR rerun builds unlinking from PRs after the PR is merged

As per the title, this is a bug, where the PR gets unlinked from the build when a build is a rerun of a build which was triggered by a PR.

Steps to reproduce:

1. Create a new PR on a branch that has pull requests builds enabled;
2. Merge the PR and close the base branch;
3. Re-run the build that was triggered by the PR;
4. The link to the PR of the build will not show up, and env variables that use the PR will not work too.

This is a problem because, as you can see on the attached image, the PR cannot be referenced within the build.