fix(superchain): image fails because ensurepip is not available

[kaizencc]

#4201 removed python3-venv, which causes superchain build to fail:

@jsii/python-runtime: ·[1G@jsii/python-runtime: $ cp ../../../README.md . && rm -f jsii-*.whl && npm run generate && npm run deps && npm run lint
@jsii/python-runtime: > @jsii/python-runtime@0.0.0 generate
@jsii/python-runtime: > ts-node build-tools/gen.ts
@jsii/python-runtime: > @jsii/python-runtime@0.0.0 deps
@jsii/python-runtime: > ts-node build-tools/deps.ts
@jsii/python-runtime: The virtual environment was not created successfully because ensurepip is not
@jsii/python-runtime: available.  On Debian/Ubuntu systems, you need to install the python3-venv
@jsii/python-runtime: package using the following command.
@jsii/python-runtime:     apt-get install python3-venv
@jsii/python-runtime: You may need to use sudo with that command.  After installing the python3-venv
@jsii/python-runtime: package, recreate your virtual environment.
@jsii/python-runtime: Failing command: ['/codebuild/output/src3494011583/src/packages/@jsii/python-runtime/.env/bin/python3', '-Im', 'ensurepip', '--upgrade', '--default-pip']
@jsii/python-runtime: /codebuild/output/src3494011583/src/packages/@jsii/python-runtime/build-tools/_constants.ts:25
@jsii/python-runtime:     throw new Error(
@jsii/python-runtime:           ^
@jsii/python-runtime: Error: Command failed with code 1: python3 -m venv /codebuild/output/src3494011583/src/packages/@jsii/python-runtime/.env

Seems like #4201 was not tested and the PR build does not build superchain.

I tested locally.

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