[ci] Explicitly declare permissions and limit credentials (#15324)

Authored by: bashonly

.github/workflows/core.yml

@@ -22,8 +22,8 @@ on:
       - yt_dlp/extractor/__init__.py
       - yt_dlp/extractor/common.py
       - yt_dlp/extractor/extractors.py
-permissions:
-  contents: read
+
+permissions: {}
 
 concurrency:
   group: core-${{ github.event.pull_request.number || github.ref }}
@@ -33,6 +33,8 @@ jobs:
   tests:
     name: Core Tests
     if: "!contains(github.event.head_commit.message, 'ci skip')"
+    permissions:
+      contents: read
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
@@ -58,6 +60,7 @@ jobs:
     - uses: actions/checkout@v6
       with:
         fetch-depth: 0
+        persist-credentials: false
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v6
       with: