[native] Change backup file separator
Summary: Turns out generateRandomString uses _ as a possible character which conflichs with the current separator so this diff changes the separator to -.
Test Plan: Generaete filenames checked that they have -.
Reviewers: marcin, kamil
Reviewed By: marcin
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D10604