Attestation is redundantly downloaded into temporary files

[benmss]

As part of the analysis process regarding attestation files, data received from the various possible sources is stored into a temporary file before being analysed. This functionality was originally created for users passing their own attestation files to Macaron at runtime, but has been reused in cases where it is not the ideal solution. Instead, attestation files that are downloaded from remote sources should be analysed while still in memory. Storage of the provenance data is already handled by the database as appropriate.