emulators/citra: unbreak fetch after 66e2037c

Failover between mirrors doesn't work if file exists but is different.

=> Attempting to fetch https://api.citra-emu.org/gamedb/?dummy=/citra/compatibility_list.json
[...]
=> SHA256 Checksum mismatch for citra/compatibility_list.json.
[...]
===>  Giving up on fetching files:  citra/compatibility_list.json

Reported by:	pkg-fallout