fix(toolkit): scrutiny dialog should with no tty

If STDIN is not connected to a TTY (terminal), and scrutiny is enabled,
we expect the program to fail (exit with non-zero exit code).

This is especially important for CI/CD scenarios where you wouldn't want
to accidentally deploy changes that didn't pass a scrutiny check.

Fixes #1380

Tested manually.

packages/aws-cdk/bin/cdk.ts

@@ -306,7 +306,13 @@ async function initCommandLine() {
       if (requireApproval !== RequireApproval.Never) {
         const currentTemplate = await readCurrentTemplate(stack);
         if (printSecurityDiff(currentTemplate, stack, requireApproval)) {
-          const confirmed = await confirm(`Do you wish to deploy these changes (y/n)?`);
+
+          // only talk to user if we STDIN is a terminal (otherwise, fail)
+          let confirmed = false;
+          if (process.stdin.isTTY) {
+            confirmed = await confirm(`Do you wish to deploy these changes (y/n)?`);
+          }
+
           if (!confirmed) { throw new Error('Aborted by user'); }
         }
       }