Upload build logs as artifacts per platform

[Copilot]

Build logs from sublibraries and the full contrib log were not being preserved as artifacts, making debugging failures difficult.

Changes

• Platform name mapping: Adds step to map matrix.os to friendly names (linux, linux_arm, macos, windows)
• Log collection: Copies and renames logs from both build directories:
  • contrib-build/contrib_build.log → contrib_build_all.log
  • contrib-build-openmp/contrib_build.log → contrib_build_openmp.log
• Artifact upload: Uses actions/upload-artifact@v4 with if: always() to ensure logs are uploaded even on failure

Artifacts are named build-logs-{platform} (e.g., build-logs-linux_arm) and automatically zipped by the action.

Original prompt

Make sure that all the logs from the different sublibraries and the full contrib log are uploaded as build artefacts. They can be zipped per build platform (linux, linux_arm ...)

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[poshul]

@jpfeuffer The logs this produces look good to me. Do you think it's good to merge?