guix: accomodate migration to codeberg

See https://guix.gnu.org/blog/2025/migrating-to-codeberg/.

When interacting with the old repo you may now also see:
```bash
warning: redirecting to https://codeberg.org/guix/guix/
```

contrib/guix/libexec/prelude.bash

@@ -50,7 +50,7 @@ fi
 # across time.
 time-machine() {
     # shellcheck disable=SC2086
-    guix time-machine --url=https://git.savannah.gnu.org/git/guix.git \
+    guix time-machine --url=https://codeberg.org/guix/guix.git \
                       --commit=53396a22afc04536ddf75d8f82ad2eafa5082725 \
                       --cores="$JOBS" \
                       --keep-failed \