ci: improve testing

Now it verifies that 廣韻.csv is built from sources in the same commit.

.github/workflows/check.yml

@@ -17,5 +17,12 @@ jobs:
       uses: actions/setup-python@v5
       with:
         python-version: '3.12'
-    - name: Check
+    - name: Verify that 廣韻.csv is built from current source
+      shell: bash
+      run: |
+        tmpdir="$(mktemp -d -p ~)"
+        mv 韻書/廣韻.csv "$tmpdir"
+        python build.py
+        diff "$tmpdir"/廣韻.csv 韻書/廣韻.csv
+    - name: Run check script
       run: python check.py