ci lint: Allow ShellCheck to test source-d scripts.

As the scripts 'utils/run-tests.sh' and 'utils/setup_test_container.sh'
use some scripts as function libraries, this change forces shellcheck to
also verify those scripts.

.github/workflows/lint.yml

@@ -88,3 +88,5 @@ jobs:
           fetch-depth: 1
       - name: Run ShellCheck
         uses: ludeeus/action-shellcheck@master
+        env:
+          SHELLCHECK_OPTS: -x

.pre-commit-config.yaml

@@ -54,4 +54,7 @@ repos:
     name: ShellCheck
     language: system
     entry: shellcheck
-    files: \.sh$
+    args: ['-x']
+    files: >
+      \.sh$
+      utils/sh*$