Remove PR ref from delete workflow

[sarahchen6]

Since README instructions recommend deleting the images after merge, the tested PR's changes will already be in master when we want to delete its test images.

Moreover, after the PR is merged, GitHub removes references to refs/pull/${{ github.event.inputs.pr_number }}/merge.

See: https://github.com/DataDog/dd-trace-java-docker-build/actions/runs/28878825640/job/85661134146

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve cleanup for unmerged variant-change PRs

When cleanup is needed before the source PR is merged or after it is closed without merging, this now checks out the default workflow ref instead of the PR ref. The create workflow still runs the PR version of scripts/create-test-mirror-entries.sh, which writes tags from that PR's CI_VARIANTS, while delete-test-mirror-entries.sh reconstructs an exact mirror.yaml block from the checked-out script and exits if it does not match. A PR that adds/removes a variant can therefore create mirror entries that this delete workflow cannot remove unless the PR has already landed; consider falling back to the PR ref when it exists or deleting by prefix rather than by the default-branch variant list.

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