GHA: use the nightly snapshots

Update to the nightly snapshots to make the CMake build easier to
maintain.

Author: compnerd

.github/workflows/pull_request.yml

@@ -40,7 +40,7 @@ jobs:
   cmake_build:
     name: CMake Build
     runs-on: ubuntu-latest
-    container: swift:6.1-noble
+    container: swiftlang/swift:nightly
     steps:
       - name: checkout sources
         uses: actions/checkout@v1