set file tag explicitly to suppress warning

[crazy-max]

related to anchore/syft#3505

https://github.com/docker/buildkit-syft-scanner/actions/runs/15877576651/job/44768951586#step:7:87

#15 [linux/amd64] generating sbom using localhost:5000/buildkit-syft-scanner:latest
#15 0.060 time="2025-06-25T13:25:09Z" level=info msg="starting syft scanner for buildkit c568c12"
#15 0.062 [0000]  WARN adding 'file' tag to the default cataloger selection, to override add '-file' to the cataloger selection request
#15 0.868 [0000]  WARN adding 'file' tag to the default cataloger selection, to override add '-file' to the cataloger selection request
#15 DONE 1.8s

[cdupuis]

LGTM