fix(ci): use env var for Docker Hub token check in workflow conditions

GitHub Actions doesn't allow secrets context in step if expressions.
Add HAS_DOCKERHUB_TOKEN env var at job level that evaluates the secret
existence, then reference that env var in step conditions.

Fixes: "Unrecognized named-value: 'secrets'" workflow validation error

.github/workflows/nightly-build.yml

@@ -65,6 +65,8 @@ jobs:
   build-and-push-nightly:
     needs: sync-development-to-nightly
     runs-on: ubuntu-latest
+    env:
+      HAS_DOCKERHUB_TOKEN: ${{ secrets.DOCKERHUB_TOKEN != '' }}
     permissions:
       contents: read
       packages: write
@@ -98,7 +100,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Log in to Docker Hub
-        if: secrets.DOCKERHUB_TOKEN != ''
+        if: env.HAS_DOCKERHUB_TOKEN == 'true'
         uses: docker/login-action@5e57cd118135c172c3672efd75eb46360885c0ef  # v3.6.0
         with:
           registry: docker.io
@@ -163,7 +165,7 @@ jobs:
 
       # Sign Docker Hub image with keyless signing (Sigstore/Fulcio)
       - name: Sign Docker Hub Image
-        if: secrets.DOCKERHUB_TOKEN != ''
+        if: env.HAS_DOCKERHUB_TOKEN == 'true'
         run: |
           echo "Signing Docker Hub nightly image with keyless signing..."
           cosign sign --yes ${{ env.DOCKERHUB_REGISTRY }}/${{ env.IMAGE_NAME }}@${{ steps.build.outputs.digest }}
@@ -171,7 +173,7 @@ jobs:
 
       # Attach SBOM to Docker Hub image
       - name: Attach SBOM to Docker Hub
-        if: secrets.DOCKERHUB_TOKEN != ''
+        if: env.HAS_DOCKERHUB_TOKEN == 'true'
         run: |
           echo "Attaching SBOM to Docker Hub nightly image..."
           cosign attach sbom --sbom sbom-nightly.json ${{ env.DOCKERHUB_REGISTRY }}/${{ env.IMAGE_NAME }}@${{ steps.build.outputs.digest }}