Cross-repo fails with `failed to load PR info`

[bkhouri]

There are occasions where the enable_cross_pr_testing: true setting on the swift_pacakge_test.yml fails with the following.

Run cat /__w/swift-package-manager/swift-package-manager/github-workflows/.github/workflows/scripts/cross-pr-checkout.swift > /tmp/cross-pr-checkout.swift
  cat /__w/swift-package-manager/swift-package-manager/github-workflows/.github/workflows/scripts/cross-pr-checkout.swift > /tmp/cross-pr-checkout.swift
  swift /tmp/cross-pr-checkout.swift "swiftlang/swift-package-manager" "10208"
  shell: sh -e {0}
Failed to load PR info from https://api.github.com/repos/swiftlang/swift-package-manager/pulls/10208: keyNotFound(CodingKeys(stringValue: "base", intValue: nil), Swift.DecodingError.Context(codingPath: [], debugDescription: "No value associated with key CodingKeys(stringValue: \"base\", intValue: nil) (\"base\").", underlyingError: nil))
Error: Process completed with exit code 1.

Here are some workflows runs:

• https://github.com/swiftlang/swift-package-manager/actions/runs/27564722105/job/81485694028?pr=10208