gh-159420: Respect BUILD_DETAILS in test_build_details

[NikitaMikhailov]

Fixes gh-159420.

CPythonBuildDetailsTests always looks for build-details.json, so it fails when CPython is configured with --with-build-details-suffix. Use sysconfig's BUILD_DETAILS value to locate the configured filename in both build-tree and installed interpreters.

Validation:

• Default build: python -m test -v test_build_details passes.
• Reconfigured and rebuilt with --with-build-details-suffix=contrib; removed the stale unsuffixed build artifact. Before the change, test_build_details reports six FileNotFoundError errors. With the change, it passes.
• Restored the original default configuration and reran test_build_details successfully.
• pre-commit checks pass for Lib/test/test_build_details.py.

This changes only the test suite, so no NEWS entry is needed.

AI assistance: Codex helped prepare the change and run validation.

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[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.