Normalize RECORD paths when writing

[hansu650]

Fixes #158.

This normalizes Windows-style separators when writing RecordEntry rows, regardless of the platform doing the write.

That keeps written RECORD paths consistent even if a RecordEntry was created from a Windows-style path on another platform.

Tests:

python -m pytest tests/test_records.py -k "backslash_path or string_representation"
python -m pytest tests/test_records.py
python -m pytest --cov=installer --cov-fail-under=100 --cov-report=term-missing --cov-context=test -n auto tests
python -m ruff check src/installer/records.py tests/test_records.py
python -m ruff format --check src/installer/records.py tests/test_records.py
python -m pre_commit run --files src/installer/records.py tests/test_records.py
git diff --check