chore(cfnspec): update npmignore to include '**/cdk.out' when creatin…

…g missing libraries (#10876)

this change was introduced in #8803 but the script to create missing construct
libraries was not updated.

As a follow-up, will look into creating a test that exercises creation of a new
construct library and the subsequent validation against pkglint.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/cfnspec/build-tools/create-missing-libraries.ts

@@ -241,6 +241,9 @@ async function main() {
       '',
       '.eslintrc.js',
       'jest.config.js',
+      '',
+      '# exclude cdk artifacts',
+      '**/cdk.out',
       'junit.xml',
       'test/',
     ]);