chore: update sha to @ syntax instead of : (#2205)

## Description

Small fix for signing in the release workflow

## Related Issue

Fixes #N/A

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [X] Other (security config, docs update, etc)

## Checklist before merging

- [X] Test, docs, adr added or updated as needed
- [X] [Contributor Guide
Steps](https://github.com/defenseunicorns/zarf/blob/main/CONTRIBUTING.md#developer-workflow)
followed

.github/workflows/release.yml

@@ -48,7 +48,7 @@ jobs:
           echo ZARF_AGENT_IMAGE_DIGEST=$(docker buildx imagetools inspect ghcr.io/defenseunicorns/zarf/agent:$GITHUB_REF_NAME --format '{{ json . }}' | jq -r .manifest.digest) >> $GITHUB_ENV
 
       - name: "Zarf Agent: Sign the Image"
-        run: cosign sign --key awskms:///${{ secrets.COSIGN_AWS_KMS_KEY }} -a release-engineer=https://github.com/${{ github.actor }} -a version=$GITHUB_REF_NAME ghcr.io/defenseunicorns/zarf/agent:$ZARF_AGENT_IMAGE_DIGEST -y
+        run: cosign sign --key awskms:///${{ secrets.COSIGN_AWS_KMS_KEY }} -a release-engineer=https://github.com/${{ github.actor }} -a version=$GITHUB_REF_NAME ghcr.io/defenseunicorns/zarf/agent@$ZARF_AGENT_IMAGE_DIGEST -y
         env:
           COSIGN_EXPERIMENTAL: 1
           AWS_REGION: ${{ secrets.COSIGN_AWS_REGION }}