Don't copy null terminators from the log's `header.gameid` into
`FifoDataFile`'s `m_game_id`.

Doing so would cause an infinite loop in `Core::GenerateScreenshotName`
as the various concatenations and `fmt::format` calls would then
effectively drop all the timestamps and disambiguating arbitrary numbers
since they followed the null terminator in the gameid, and so the
`File::Exists` calls would always return true.

Fixes https://bugs.dolphin-emu.org/issues/14002.

Previously, SDL would `find_package(libusb)` which would actually overwrite the user preference in the case where both USE_SYSTEM_LIBUSB and USE_SYSTEM_SDL were OFF. This coincidentally also allows SDL to use libusb on Windows.

Our FindLibUSB.cmake was previously entirely unused unless SDL was being built from Externals, we now rely on it again. It will use PkgConfig if applicable or fall back to looking around on the system, and more importantly it will always create an imported target.

cURL tries to find ZSTD anyways due to the lack of setting it in CACHE, and for both ZLIB and ZSTD if we had done find_package previously, cURL would still use ZLIB/ZSTD regardless of the set variable.

This also fixes the CMake build on Windows, since it would fail to find our target defined in Externals and would fall back to trying to redefine it, which produces an error.