Move Project Export \ Import process away from using 'shared_path' by default

Noticed today while testing that the Project Import process was using a configured NFS mount as a temp location for expanding the tarballs.

After further investigation it appears the process is using the gitlab_rails['shared_path'] for this. This path will typically be set to NFS mounts so that multi-node setups can share files. However with the general efforts to move away from NFS across GitLab reliance on this path is being reduced as data is moved into Object Storage.

Imports are an interesting one in that we'll always likely need a local path to expand the tarball so suggestion is to move to a local temp path instead.