fix(lambda-python): Pipenv projects no longer work for Python 3.6 (#20019)

`pipenv` has dropped support for Python 3.6. Our install command was
installing the latest `pipenv`, but that will start failing. Instead,
pin to the latest version that still supports 3.6.


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

Author: rix0rrr

packages/@aws-cdk/aws-lambda-python/lib/Dockerfile

@@ -9,6 +9,8 @@ ARG HTTPS_PROXY
 
 # Upgrade pip (required by cryptography v3.4 and above, which is a dependency of poetry)
 RUN pip install --upgrade pip
-RUN pip install pipenv poetry
+
+# pipenv 2022.4.8 is the last version with Python 3.6 support
+RUN pip install pipenv==2022.4.8 poetry
 
 CMD [ "python" ]