Fix PHP8 pipeline

As per https://github.com/PrivateBin/PrivateBin/pull/843#issuecomment-939526915

Co-Authored-By: El RIDO <elrido@gmx.net>

.github/workflows/refresh-php8.yml

@@ -12,10 +12,14 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
-
       - name: Checkout php8 branch 
-        run: git checkout php8
+        uses: actions/checkout@v2
+        with:
+          # directly checkout the php8 branch
+          ref: php8
+          # Number of commits to fetch. 0 indicates all history for all branches and tags.
+          # Default: 1
+          fetch-depth: 0
 
       - name: Merge master changes into php8
         run: git merge master