Support PRs to use PR ID in the artifact name

[mdelapenya]

When packaging a PR, the generated artifact is uploaded to a Google Cloud bucket with the name of the snapshot. We'd like to generate the artifact including the PR ID:

- gs://beats-ci-artifacts/snapshots/elastic-agent-8.0.0-SNAPSHOT-linux-x86_64.tar.gz
+ gs://beats-ci-artifacts/pull-requests/elastic-agent-8.0.0-SNAPSHOT-${PR_ID}-linux-x86_64.tar.gz

cc/ @elastic/observablt-robots @ph @EricDavisX

[mdelapenya]

Revisiting the mage build tools, and to avoid refactoring it to support creating the packages with a different name, I think we can use another schema for the bucket layout:

gs://beats-ci-artifacts/pull-requests/pr-123/elastic-agent-8.0.0-SNAPSHOT-linux-x86_64.tar.gz

@kuisathaverat wdyt?

[ph]

@mdelapenya +1 for not changing the artifact names, the Elastic-Agent relies on that quite heavily and with the upgrade this will only increase.

@michalpristas and @blakerouse for info.