Update Harper CLI installation method in workflow

Author: Amanda-dong

.github/workflows/harper.yml

@@ -14,7 +14,7 @@ jobs:
         uses: actions/checkout@v6
 
       - name: Install Harper CLI
-        run: cargo install harper-cli --locked
+        run: cargo install --git https://github.com/Automattic/harper harper-cli --locked
 
       - name: Run Harper
         run: harper-cli lint docs src/pages