scripts: runners: fix openocd on Windows

Commit 3124c029 ("cmake: flash/debug: refactor runner configuration")
changed the way the hex_file and elf_file inputs in the RunnerConfig
object are created. In particular, they are now host-style paths.

This breaks flashing with openocd on Windows, which doesn't handle that
properly. Fix that by "casting" the internal hex_file and elf_file
attributes to POSIX paths.

Fixes: #28138
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>