Run clang-format-check.yml on pull_request

It is inconvenient to report a formatting error after a pull request has
been merged.

.github/workflows/clang-format-check.yml

diff --git a/.github/workflows/clang-format-check.yml b/.github/workflows/clang-format-check.yml
index 080bb74..6448fc5 100644
--- a/.github/workflows/clang-format-check.yml
+++ b/.github/workflows/clang-format-check.yml
@@ -1,5 +1,5 @@
 name: CI Format Check
-on: [push]
+on: [push, pull_request]
 
 permissions:
   contents: read