chore(lambda): replace rsync with cp in bundling integ test (#9530)

`rsync` is no longer present in the latest
`amazon/aws-sam-cli-build-image-python3.6` image.

This is similar to #9355 


----

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

packages/@aws-cdk/aws-lambda/README.md

@@ -362,7 +362,7 @@ new lambda.Function(this, 'Function', {
       command: [
         'bash', '-c', `
         pip install -r requirements.txt -t /asset-output &&
-        rsync -r . /asset-output
+        cp -au . /asset-output
         `,
       ],
     },

packages/@aws-cdk/aws-lambda/test/integ.bundling.ts

@@ -20,7 +20,7 @@ class TestStack extends Stack {
           image: lambda.Runtime.PYTHON_3_6.bundlingDockerImage,
           command: [
             'bash', '-c', [
-              'rsync -r . /asset-output',
+              'cp -au . /asset-output',
               'cd /asset-output',
               'pip install -r requirements.txt -t .',
             ].join(' && '),