Temporarily lock SFML revision used in CI

The SFML 3 API is changing faster than I care to update in CSFML.
For now let's lock CSFML to a particular revision of SFML until
the SFML 3 API changes slow down and we can go back to tracking
the master branch.

.github/workflows/ci.yml

@@ -46,7 +46,7 @@ jobs:
       uses: actions/checkout@v4
       with:
         repository: SFML/SFML
-        ref: master
+        ref: c89c32d7baa15927a39a161a4ead867c11d5ea21
         path: SFML
 
     - name: Configure SFML CMake