Inconsistent handling of override file paths with/without a directory

Environments:

• Prettier Version: 3.9.6
• Running Prettier via: CLI
• Runtime: Node.js v24.19.0
• Operating System: NixOS
• Prettier plugins (if any): N/A

Steps to reproduce:

1. Clone https://github.com/l0b0/prettier-bug-test
2. Run ./succeeds.bash and note that nothing changed
3. Run ./fails.bash

Expected behavior:

Nothing should change, because the location of the configuration change should not affect the result.

Actual behavior:

Override paths with a directory separator are interpreted as relative to the configuration file, rather that the current working directory. That is, config/dir/regular.md and dir/exception.md both get formatted.