feat: implement replacing previous comments

Based on https://github.com/peter-evans/find-comment

action.yml

@@ -67,10 +67,51 @@ outputs:
 runs:
   using: 'composite'
   steps:
+    - name: Find PR (if it exists)
+      id: pr-number
+      if: inputs.comment-on-pr == 'true'
+      run: |
+        . "$GITHUB_ACTION_PATH/utils.sh"
+
+        log 'Determine head_ref'
+        # For push & tag events it'll bet GITHUB_REF_NAME, for pull_request events it'll be GITHUB_HEAD_REF
+        head_ref=${GITHUB_REF_NAME:-$GITHUB_HEAD_REF}
+
+        log "Get PR number for $head_ref"
+        prs=$(curl -X 'GET' \
+                "$GITHUB_API_URL/repos/$GITHUB_REPOSITORY/pulls?state=open&sort=recentupdate" \
+                -H "Authorization: token $GITHUB_TOKEN" \
+                -H 'accept: application/json')
+
+        pr_number=$(echo "$prs" |
+                jq --arg head_ref "$head_ref" '.[] | select(.head.ref == $head_ref) | .number')
+
+        # Protect against running before a PR is made or if it is triggered on the main branch
+        if [ -z "$pr_number" ]; then
+                warn "No PR created for this commit"
+                echo "pr-number=" >> "$GIHUB_OUTPUT"
+                exit 0
+        fi
+
+        log "Retrieved index: $pr_number"
+        log "Expected PR URL: $GITHUB_SERVER_URL/$GITHUB_REPOSITORY/pulls/$pr_number"
+
+        echo "pr-number=$pr_number" >> "$GIHUB_OUTPUT"
+    - name: Find previous comment (if present)
+      # We want to generate a comment, and we we able to fin the PR number
+      if: inputs.comment-on-pr == 'true' && steps.pr-number.outputs.pr-number != ''
+      id: find-comment
+      uses: https://github.com/peter-evans/find-comment@3eae4d37986fb5a8592848f6a574fdf654e61f9e # v3
+      with:
+        issue-number: steps.pr-number.outputs.pr-number
+        direction: first
+        body-includes: "<!-- AUTOGENERATED by nix-flake-outputs-size action -->"
     - name: Create report
       if: inputs.comment-on-pr == 'true' || inputs.generate-artifact == 'true'
       env:
+        PR_ID: ${{ steps.pr-number.outputs.pr-number }}
         COMMENT: ${{ inputs.comment-on-pr }}
+        COMMENT_ID: ${{ steps.find-comment.outputs.comment-id }}
         ARTIFACT_NAME: ${{ inputs.artifact-name }}
         DO_COMPARISON: ${{ inputs.do-comparison }}
         BASE_BRANCH: ${{ inputs.base-branch }}