fix(cli): cannot set environment variable CI=false (#32749)

We check the `$CI` environment variable for its presence, not for its value. This means that setting `CI=false` enables CI mode instead of disabling it.

This is unexpected. Fix it.

We could have checked for `CI=true` or `CI=1`, but the current behavior is backwards compatible with crazy things users might already be doing.


----

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

packages/aws-cdk/lib/util/yargs-helpers.ts

@@ -27,7 +27,7 @@ export function yargsNegativeAlias<T extends { [x in S | L]: boolean | undefined
  * @returns true if the current process is running in a CI environment
  */
 export function isCI(): boolean {
-  return process.env.CI !== undefined;
+  return process.env.CI !== undefined && process.env.CI !== 'false' && process.env.CI !== '0';
 }
 
 /**

packages/aws-cdk/test/util/yargs-helpers.test.ts

@@ -0,0 +1,14 @@
+import { isCI } from '../../lib/util/yargs-helpers';
+
+test.each([
+  ['true', true],
+  ['1', true],
+  ['false', false],
+  ['0', false],
+  // The following ones are unexpected but this is the legacy behavior we're preserving.
+  ['banana', true],
+  ['', true],
+])('test parsing of falsey CI values: %p parses as %p', (envvar, ci) => {
+  process.env.CI = envvar;
+  expect(isCI()).toEqual(ci);
+});