ci: ignore error if gha cache export fails

[crazy-max]

We often encounter rate limits when trying to export the GHA cache on this repo:

• https://github.com/moby/buildkit/actions/runs/12761713702/job/35574523031?pr=5649#step:7:1948
• https://github.com/moby/buildkit/actions/runs/12763721138/job/35575847332?pr=5638#step:8:1551
• https://github.com/moby/buildkit/actions/runs/12763721107/job/35574441704#step:4:854

#41 exporting to GitHub Actions Cache
#41 preparing build cache for export
#41 writing layer sha256:4dfab37fddd981de09c236979ac9e012419dc3b5abc78e9b416e5ac243838826
#41 writing layer sha256:4dfab37fddd981de09c236979ac9e012419dc3b5abc78e9b416e5ac243838826 0.1s done
#41 writing layer sha256:bcf9cb26c40f8c5b74030342ac94a99169912038ab09dc3d727e1c7452c6ac7a
#41 writing layer sha256:bcf9cb26c40f8c5b74030342ac94a99169912038ab09dc3d727e1c7452c6ac7a 0.1s done
#41 preparing build cache for export 607.5s done
#41 ERROR: maximum timeout reached

We can use the ignore-error attribute to avoid jobs to fail. We also have the testBasicGhaCacheImportExportExtraTimeout integration test for such case so I'm not that concerned that we could miss actual issues with GHA cache backend.