Update tests.yml

Author: ikesyo

.github/workflows/tests.yml

@@ -45,7 +45,9 @@ jobs:
       run: |
         # https://github.com/giginet/Scipio/issues/239
         # https://github.com/giginet/Scipio/issues/241
-        git checkout https://github.com/firebase/firebase-ios-sdk 11.15.0
+        git clone https://github.com/firebase/firebase-ios-sdk \
+          -b 11.15.0 \
+          --depth=1
         swift run scipio create firebase-ios-sdk \
           --framework-type static \
           --platform iOS