Build/Test Tools: Account for missing performance artifacts on older branches

[ArkaPrabhaChowdhury]

What?

Updates the performance workflow so missing previous-build artifacts only fail the workflow for runk and the latest release branch.

For older branches, the workflow now logs a warning and skips the previous-build comparison instead of failing.

Why?

Build artifacts for older branches are more likely to expire or be missing due to long gaps between commits. In those cases, failing the workflow is too strict.

Trac ticket: https://core.trac.wordpress.org/ticket/63617
Contribution tracking issue: https://github.com/rtCamp/wp-contributions/issues/2953

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