build: fix GITHUB_OUTPUT multiline variable error (#400)

This fixes the provenance generation output to properly write multi-line
string to GITHUB_OUTPUT for use in subsequent jobs.

.github/actions/publish/action.yml

@@ -68,8 +68,8 @@ runs:
  image_and_digest=$(echo "$ARTIFACTS" | jq -r '.[] | select (.type=="Docker Manifest") | .path')
  image=$(echo "${image_and_digest}" | cut -d'@' -f1 | cut -d':' -f1)
  digest=$(echo "${image_and_digest}" | cut -d'@' -f2)
- { echo 'name<<EOF'; echo $image; echo EOF } >> "$GITHUB_OUTPUT"
- { echo 'digest<<EOF'; echo $digest; echo EOF } >> "$GITHUB_OUTPUT"
+ { echo 'name<<END'; echo $image; echo END; } >> "$GITHUB_OUTPUT"
+ { echo 'digest<<END'; echo $digest; echo END; } >> "$GITHUB_OUTPUT"
 
  - name: Upload Release Artifacts
  shell: bash