build_master_dev.yml drop redundant cosign privkey save step

Author: vszakats

.github/workflows/build_master_dev.yml

@@ -86,8 +86,6 @@ jobs:
         name: 'push images to github registry'
       - name: Install Cosign
         uses: sigstore/cosign-installer@d7543c93d881b35a8faa02e8e3605f69b7a1ce62 # v3.10.0
-      - name: Write signing key to disk (only needed for `cosign sign --key`)
-        run: echo "${{ secrets.COSIGN_PRIVATE_KEY }}" > cosign.key
       - name: Sign image with a key
         run: |
           cosign sign -y --key cosign.key ghcr.io/curl/curl-container/curl-dev-fedora:master