`actions/upload-artifact@v4` comes with the following [breaking
change](https://github.com/actions/upload-artifact?tab=readme-ov-file#breaking-changes):
"Due to how Artifacts are created in this new version, it is no longer
possible to upload to the same named Artifact multiple times. You must
either split the uploads into multiple Artifacts with different names,
or only upload once. Otherwise you will encounter an error."
Due to that we cannot copy multiple blob report folders into the same
artifact name and rely on the action to merge them. Instead, as
suggested by their migration guide, we upload each blob report into a
uniquely named artifact with prefix `blob-report-` and then download all
of them into same directory.
This version change also affects how we store pull_request_number.txt
into an artifact. Previously we relied on the fact that uploading
artifact with the same name would silently override existing one, but
now it's an error. To overcome that, we upload PR number file into
uniquely named artifacts `pull-request-*` and later extract them into
same location with `unzip -n` which will never override existing file,
so we end up with single `pull_request_number.txt`.
Reference #28800
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