Python 3.14: Use relative paths in build-details.json

[geofft]

PEP 739 specifies a build-details.json file, which is expected to be present for Python 3.14 and up.

The CPython build process automatically creates this but it uses absolute paths:

$ head ~/.local/share/uv/python/cpython-3.14.0b4-linux-x86_64-gnu/lib/python3.14/build-details.json 
{
  "schema_version": "1.0",
  "base_prefix": "/install",
  "base_interpreter": "/install/bin/python3.14",
  "platform": "",
  "language": {
    "version": "3.14",
    "version_info": {
      "major": 3,
      "minor": 14,

The PEP permits a path relative to build-details.json for base_prefix and paths relative to base_prefix for all other fields. We should switch to using relative paths.

I haven't yet looked to see if this is a knob we can turn on at build time or we need to postprocess it. If there is not such a knob we should ideally send a patch to upstream CPython to add one, perhaps as part of a generic ./configure --relocatable.