adding merge queue support to PR build workflow with redundant build skipping

[pulimsr]

Adds merge queue support to the PR build workflow. When a PR enters the merge queue, it compares the merge group parent SHA against the PR's base SHA to detect if a rebuild is needed — skipping redundant builds when the PR is first in queue and the base hasn't moved.

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• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
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