nx_tzdb: check for unpacked directory

Otherwise things get funny if the archive is downloaded, but the unpacking was interrupted.

externals/nx_tzdb/CMakeLists.txt

@@ -32,7 +32,7 @@ set(NX_TZDB_ARCHIVE "${CMAKE_CURRENT_BINARY_DIR}/${NX_TZDB_VERSION}.zip")
 
 set(NX_TZDB_ROMFS_DIR "${CMAKE_CURRENT_BINARY_DIR}/nx_tzdb")
 
-if ((NOT CAN_BUILD_NX_TZDB OR YUZU_DOWNLOAD_TIME_ZONE_DATA) AND NOT EXISTS ${NX_TZDB_ARCHIVE})
+if ((NOT CAN_BUILD_NX_TZDB OR YUZU_DOWNLOAD_TIME_ZONE_DATA) AND NOT EXISTS ${NX_TZDB_ROMFS_DIR})
     set(NX_TZDB_DOWNLOAD_URL "https://github.com/lat9nq/tzdb_to_nx/releases/download/${NX_TZDB_VERSION}/${NX_TZDB_VERSION}.zip")
 
     message(STATUS "Downloading time zone data from ${NX_TZDB_DOWNLOAD_URL}...")