Fix OS.get_system_font_path and OS.get_system_font_path_for_text to return correct slashes

[sepTN]

Fix #86544

The bug also occurs in OS.get_system_font_path_for_text so I've fixed that as well.

Snippet used:

func _ready() -> void:
	print(OS.get_system_font_path("sans-serif"))
	print(OS.get_system_font_path("serif"))
	print(OS.get_system_font_path("monospace"))
	print(OS.get_system_font_path("cursive"))
	print(OS.get_system_font_path("fantasy"))

Print result:

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.2.2.