upgrade actions/cache to v4 as v2 failed

Author: maggick

.github/workflows/pelican.yml

@@ -30,7 +30,7 @@ jobs:
         run: echo "::set-output name=dir::$(pip cache dir)"
 
       - name: Cache dependencies
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: ${{ steps.pip-cache.outputs.dir }}
           key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}