file-cache: Fix bug when updating tiles

When store_tile() was called and the tile already existed in the cache,
it would fail to overwrite it, because it was using
g_file_create_async() instead of g_file_replace_async().
1 job for file-cache-bug in 2 minutes and 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1243988
fedora

00:02:06

51.0%