ci: fix Flox installation action reference

- Update flox/install-flox@v1 to determinatesystems/flox-install-action@v1
- This fixes the CI failure in the flox-build job

Co-Authored-By: michael@flox.dev <michael@flox.dev>

.github/workflows/ci.yml

@@ -35,7 +35,7 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Install Flox
-        uses: flox/install-flox@v1
+        uses: determinatesystems/flox-install-action@v1
 
       - name: Build with Flox
         run: |