ci(shellcheck): fix exit code propagation

Return properly the exit code status of shellcheck command when looping
through shell script files.

run-tests.sh

@@ -76,7 +76,7 @@ check_commitlint () {
 }
 
 check_shellcheck () {
-    find . -name "*.sh" -exec shellcheck {} \;
+    find . -name "*.sh" -exec shellcheck {} \+
 }
 
 check_pydocstyle () {