build: ensure pull request number is set when applicable

---
type: pre_commit_static_analysis_report
description: Results of running static analysis checks when committing changes.
report:
  - task: lint_filenames
    status: passed
  - task: lint_editorconfig
    status: passed
  - task: lint_markdown
    status: na
  - task: lint_package_json
    status: na
  - task: lint_repl_help
    status: na
  - task: lint_javascript_src
    status: na
  - task: lint_javascript_cli
    status: na
  - task: lint_javascript_examples
    status: na
  - task: lint_javascript_tests
    status: na
  - task: lint_javascript_benchmarks
    status: na
  - task: lint_python
    status: na
  - task: lint_r
    status: na
  - task: lint_c_src
    status: na
  - task: lint_c_examples
    status: na
  - task: lint_c_benchmarks
    status: na
  - task: lint_c_tests_fixtures
    status: na
  - task: lint_shell
    status: na
  - task: lint_typescript_declarations
    status: na
  - task: lint_typescript_tests
    status: na
  - task: lint_license_headers
    status: passed
---

Author: Planeshifter

.github/workflows/first_time_greeting.yml

@@ -56,7 +56,7 @@ jobs:
       # Greet first-time contributors:
       - name: 'Greet first-time contributors'
         env:
-          ISSUE_NUMBER: ${{ github.event.issue.number }}
+          ISSUE_NUMBER: ${{ github.event.issue.number || github.event.pull_request.number }}
           GITHUB_TOKEN: ${{ secrets.STDLIB_BOT_PAT_REPO_WRITE }}
         run: |
           . "$GITHUB_WORKSPACE/.github/workflows/scripts/first_time_greeting" $ISSUE_NUMBER