Simplify CI build job Python setup

[psamanoelton]

Summary

This PR simplifies the Python setup used by the main Bazel build job in CI.

Previously, the self-hosted/container-based build job relied on actions/setup-python, and the workflow had to manually copy Python headers into the hosted-toolcache layout so Bazel's @system_python repository could find Python.h correctly. This PR removes that workaround by using container-managed Python 3.10 directly inside the build job.

The goal is to keep the same Bazel build/test/package behavior while making the CI setup simpler and easier to maintain.

What changed

• removed actions/setup-python from the main build job
• installed Python 3.10, python3.10-dev, and python3.10-venv directly in the container
• created a local virtualenv for the job and used that interpreter for pip/Bazel-related steps
• removed the manual Python header-copy workaround
• removed the temporary debug steps that were only needed to diagnose the hosted-toolcache/header mismatch
• kept the rest of the build/test/package flow unchanged

Validation

This change should be validated against the same CI behaviors as before:

• bazel fetch //tensorboard/...
• bazel build //tensorboard/...
• bazel test //tensorboard/...
• bazel test //tensorboard/... --test_tag_filters="support_notf"
• bazel build //tensorboard/pip_package:test_pip_package
• ./bazel-bin/tensorboard/pip_package/test_pip_package --tf-version tf-nightly
• ./bazel-bin/tensorboard/pip_package/test_pip_package --tf-version notf