local: fix attestation filename

[crazy-max]

fixes #6049

FROM alpine AS base
ARG TARGETARCH
ARG BUILDKIT_SBOM_SCAN_STAGE=true
RUN apk add git
COPY <<EOF /out/empty_${TARGETARCH}
EOF

FROM scratch
COPY --from=base /out /

$ docker buildx build --sbom=true --output type=local,dest=./bin . && tree ./bin
./bin
├── empty_amd64
├── sbom-base.spdx.json
└── sbom.spdx.json

$ docker buildx build --sbom=true --output type=local,dest=./bin,platform-split=false . && tree ./bin
./bin
├── empty_amd64
├── sbom-base.spdx.json
└── sbom.spdx.json

$ docker buildx build --sbom=true --output type=local,dest=./bin,platform-split=true . && tree ./bin
./bin
└── linux_amd64
    ├── empty_amd64
    ├── sbom-base.spdx.json
    └── sbom.spdx.json

$ docker buildx build --sbom=true --platform=linux/amd64,linux/arm64 --output type=local,dest=./bin . && tree ./bin
./bin
├── linux_amd64
│   ├── empty_amd64
│   ├── sbom-base.spdx.json
│   └── sbom.spdx.json
└── linux_arm64
    ├── empty_arm64
    ├── sbom-base.spdx.json
    └── sbom.spdx.json

$ docker buildx build --sbom=true --platform=linux/amd64,linux/arm64 --output type=local,dest=./bin,platform-split=false . && tree ./bin
./bin
├── empty_amd64
├── empty_arm64
├── sbom-base.spdx.linux_amd64.json
├── sbom-base.spdx.linux_arm64.json
├── sbom.spdx.linux_amd64.json
└── sbom.spdx.linux_arm64.json

$ docker buildx build --sbom=true --platform=linux/amd64,linux/arm64 --output type=local,dest=./bin,platform-split=true . && tree ./bin
./bin
├── linux_amd64
│   ├── empty_amd64
│   ├── sbom-base.spdx.json
│   └── sbom.spdx.json
└── linux_arm64
    ├── empty_arm64
    ├── sbom-base.spdx.json
    └── sbom.spdx.json

[tonistiigi]

Could be better to use a different name in here, eg. addPlatformToName, as this is slightly different from the platforms being split or not.