ci: avoid hardcoded dexidp GitHub org

Two changes here, the ghcr.io image is made  using the github.repository
variable so the attestation should be as well. The Docker Hub image
should only be built when the repo matches the upstream official repo.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>

.github/workflows/artifacts.yaml

@@ -75,7 +75,7 @@ jobs:
         with:
           images: |
             ${{ steps.image-name.outputs.value }}
-            dexidp/dex
+            ${{ github.repository == 'dexidp/dex' && 'dexidp/dex' || '' }}
           flavor: |
             latest = false
           tags: |
@@ -186,7 +186,7 @@ jobs:
       - name: Generate build provenance attestation
         uses: actions/attest-build-provenance@e8998f949152b193b063cb0ec769d69d929409be # v2.4.0
         with:
-          subject-name: ghcr.io/dexidp/dex
+          subject-name: ghcr.io/${{ github.repository }}
           subject-digest: ${{ steps.build.outputs.digest }}
           push-to-registry: true
         if: inputs.publish