[build.ps1] use Python 3.12+ from Python.org

[stevapple]

This PR bumped default Python to 3.12.6 and switched to use Python.org installers. Downloading from Python.org can utilize their SBOM for checksum, so all releases after 3.12 will be allowed.

This PR also addressed the regression where Python 3.12 doesn't have the distutils (and packaging, for the upcoming rebranch) modules required for building LLDB. They're now directly installed from trusted wheels.

These changes make the cached Python fully capable of handling the whole build process, so host Python is no longer required.

[compnerd]

Can you check what python version is included in the latest Xcode 16 release? We should try to maintain that parity there as we currently are in sync (3.9).

[compnerd]

@stevapple could you pull out part of the change you have here? You are introducing a Get-PythonExecutable which is a really nice improvement that doesn't need to be gated on the version change. I'd love to see that merged early.

[stevapple]

We should try to maintain that parity there as we currently are in sync (3.9).

Is there any specific benefit to do that? On Linux we’re always sticking to the system-provided python3 version, which is already 3.12 on several distributions (eg. Ubuntu 22.04). Given the support lifecycle of Python and the state of Python 3.9 on Windows (deprecated in VS and NuGet, unavailable for WoA), we should be upgrading to at least 3.11.

[stevapple]

@stevapple could you pull out part of the change you have here?

Will do that later today.