ci: Move step output to env var

Author: Techassi

.github/workflows/ubi-rust-builder.yml

@@ -46,11 +46,13 @@ jobs:
           tags: oci.stackable.tech/sdp/${{ matrix.ubi-version }}-rust-builder:${{ env.TAG }}
       - name: Sign the published builder image
         shell: bash
+        env:
+          DIGEST: ${{ steps.build-and-push.outputs.digest }}
         run: |
           # Refer to image via its digest (oci.stackable.tech/sdp/airflow@sha256:0a1b2c...)
           # This generates a signature and publishes it to the registry, next to the image
           # Uses the keyless signing flow with Github Actions as identity provider
-          cosign sign -y "oci.stackable.tech/sdp/${{ matrix.ubi-version }}-rust-builder@${{ steps.build-and-push.outputs.digest }}"
+          cosign sign -y "oci.stackable.tech/sdp/${{ matrix.ubi-version }}-rust-builder@$DIGEST"
   create_manifest:
     permissions:
       id-token: write