Improved SBOM filenames

[jedevc]

From the following Dockerfile:

ARG BUILDKIT_SBOM_SCAN_STAGE=true

FROM alpine:latest AS base
RUN echo test > /test

FROM scratch
COPY --from=base /test /

we get:

$ docker buildx build --builder=dev . --sbom=true --provenance=true -o ./dir
$ ls ./dir
provenance.json  sbom-base.spdx.json  sbom.spdx.json  foo