ci: make sure dictionary words are sorted and unique (#6316)

.github/workflows/ci.yml

@@ -1011,6 +1011,19 @@ jobs:
         with:
           tool: cargo-spellcheck
       - uses: actions/checkout@v4
+      - name: Make sure dictionary words are sorted and unique
+        run: |
+          # `sed` removes the first line (number of words) and
+          # the last line (new line). 
+          # 
+          # `sort` makes sure everything in between is sorted
+          # and contains no duplicates.
+          # 
+          # Since `sort` is sensitive to locale, we set it
+          # using LC_ALL to en_US.UTF8 to be consistent in different
+          # environments.
+
+          sed '1d; $d' spellcheck.dic | LC_ALL=en_US.UTF8 sort -uc
       - name: Run cargo-spellcheck
         run: cargo spellcheck --code 1