fix: truncated 'cdk diff' output in pipes

We are force-exiting the process with `process.exit(1)`, which may leave
some `stdout` output unflushed. This is especially visible when doing
`cdk diff | tee diff.txt`, if there is a large diff. The output will
be truncated halfway through.

The correct solution is to set the exit code and let NodeJS terminate
itself gracefully when all output has been flushed.

packages/aws-cdk/bin/cdk.ts

@@ -387,11 +387,11 @@ initCommandLine()
     if (typeof value === 'string') {
       data(value);
     } else if (typeof value === 'number') {
-      process.exit(value);
+      process.exitCode = value;
     }
   })
   .catch(err => {
     error(err.message);
     debug(err.stack);
-    process.exit(1);
+    process.exitCode = 1;
   });