fetcher: Always open tmpfiles in repo (except on FUSE)

fetcher: Always open tmpfiles in repo (except on FUSE)

This reverts commit 4e61e6f7d0d6aebd6abcdc455ec53164afe39e8d
and re-instates the fix for ensuring that we download temporary
files into the repository location.

However in order to ensure we don't re-introduce
https://github.com/ostreedev/ostree/issues/2900
we detect the case where we're writing to a FUSE mount
and keep the prior behavior.

I've verified that this works with flatpak.

Note a downside of this is the change needs to be triplicated
across the 3 http backends.

This then again
Closes: https://github.com/ostreedev/ostree/issues/2571