[GitHub][docker] Add python3 venv package to CI container (llvm#161024)

I'm trying to make `pr-code-format.yml` job run natively on
`ci-ubuntu-24.04` container.
As it appears, `ci-ubuntu-24.04` already
[has](https://github.com/llvm/llvm-project/blob/41a2dfc0d77d9ad977d1d36358f979abb3a0928f/.github/workflows/containers/github-action-ci/Dockerfile#L35)
latest `clang-format`, `python3.12` installed, but `python3.12` needs
`venv` to work properly, and Ubuntu asks for `python3-venv` package to
be installed to create a venv.

Author: vbvictor

.github/workflows/containers/github-action-ci/Dockerfile

@@ -62,6 +62,7 @@ RUN apt-get update && \
     # Having a symlink from python to python3 enables code sharing between
     # the Linux and Windows pipelines.
     python3-pip \
+    python3-venv \
     file \
     tzdata \
     python-is-python3 && \