ci: use Eden workflow from 1.0.15 tag

[europaul]

Description

Switch all Eden test workflow references to the newest version 1.0.15.
It includes updated runner selection logic that uses the self-hosted
nested-virt runners.

Test compatibility is preserved since the actual test definitions are controlled
by the eden_version input parameter, which remains pinned per stable branch.

We'll only see the effects of this PR after merging into master, since Eden workflow is always triggered from master.

How to test and validate this PR

• Verify Eden tests trigger correctly on a PR targeting master
• Verify Eden tests trigger correctly on a PR targeting a stable branch (e.g. 14.5-stable)
• Confirm jobs pick up self-hosted nested-virt runners instead of buildjet

Changelog notes

No user-facing changes.

PR Backports

• 16.0-stable: No, CI workflow is not shared across branches.
• 14.5-stable: No, CI workflow is not shared across branches.
• 13.4-stable: No, CI workflow is not shared across branches.

Checklist

• I've provided a proper description
• I've added the proper documentation
• I've tested my PR on amd64 device
• I've tested my PR on arm64 device
• I've written the test verification instructions
• I've set the proper labels to this PR

And the last but not least:

• I've checked the boxes above, or I've provided a good reason why I didn't
  check them.

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