Switch shellcheck soundness check to a lighter weight image

This just installs and runs shellcheck. It doesn't need a 1 GB Swift toolchain image, use the much smaller slim variant. This makes shellcheck run faster and doesn't waste resources.

Author: jakepetroules

.github/workflows/soundness.yml

@@ -65,8 +65,8 @@ on:
         default: true
       shell_check_container_image:
         type: string
-        description: "Container image for the shell check job. Defaults to latest Swift Ubuntu image."
-        default: "swift:6.2-noble"
+        description: "Container image for the shell check job. Defaults to latest (slim) Swift Ubuntu image."
+        default: "swift:6.2-noble-slim"
       yamllint_check_enabled:
         type: boolean
         description: "Boolean to enable the YAML lint check job. Defaults to true."