Change bookworm-slim to stable-slim for pure-rust-build

With the "Verify environment is sufficiently minimal for the test"
step, if a new stable release of Debian has software that makes the
environment too extensive for `pure-rust-build` to produce accurate
results, then that should be discovered.

Author: EliahKagan

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ on:
 jobs:
   pure-rust-build:
     runs-on: ubuntu-latest
-    container: debian:bookworm-slim
+    container: debian:stable-slim
     steps:
       - uses: actions/checkout@v4
       - name: Prerequisites