ci(shellcheck): fix exit code propagation (reanahub#29)

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

run-tests.sh

@@ -28,7 +28,7 @@ check_commitlint () {
 }
 
 check_shellcheck () {
-    find . -name "*.sh" -exec shellcheck {} \;
+    find . -name "*.sh" -exec shellcheck {} \+
 }
 
 check_all () {