Fix posting reproducibility index to PR

The current version does not work because when running on merge events
the PR number is not present, so we need to look it up based on commit
SHA.

Ref #861

.github/workflows/reproducibility.yaml

@@ -44,11 +44,25 @@ jobs:
           github-token: ${{secrets.GITHUB_TOKEN}}
           script: |
             const fs = require('fs').promises;
-            file_content = await fs.readFile('./reproducibility_index');
+            const file_content = await fs.readFile('./reproducibility_index');
+
+            const opts = await github.repos.listPullRequestsAssociatedWithCommit({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              commit_sha: context.sha
+            });
+            // See:
+            // - https://octokit.github.io/rest.js/v17#previews
+            // - https://developer.github.com/v3/repos/commits/#list-pull-requests-associated-with-commit
+            opts.mediaType = {
+                previews: ['groot']
+            };
+
+            const issues = await github.paginate(opts);
 
             await github.issues.createComment({
-              issue_number: context.issue.number,
+              issue_number: issues[0].number,
               owner: context.repo.owner,
               repo: context.repo.repo,
               body: 'Reproducibility index:\n\n```\n' + file_content + '\n```\n'
-            })
+            });