fix(cli): program execution fails without debug information

When executing the CDK app program fails, we don't print any useful debug information. This makes sense because we are passing all output from the program to the shell, expecting this would be enough to debug any faults. However the program might be faulty in a way that no (useful) output is printed. To help with this case print the failing command when `--debug` is enabled.

This might require a follow up with a better DX for the generic non debug case. For now this will improve the situation.

packages/aws-cdk/lib/api/cxapp/exec.ts

@@ -118,6 +118,7 @@ export async function execProgram(aws: SdkProvider, config: Configuration): Prom
         if (code === 0) {
           return ok();
         } else {
+          debug('failed command:', commandAndArgs);
           return fail(new Error(`Subprocess exited with error ${code}`));
         }
       });