Image artifacts retain some permissions from the source file

[jrobble]

The artifact generated for image jobs (one file per image) start with permissions of the source file. This is because we copy the original image over to the artifact location. Those permissions are then masked by the "mpf" umask. To be clear, when I say "masked" I mean that permissions are removed from the file that don't appear in the umask. I ran a test locally and verified this is the case.

This can result in some of the image artifacts having executable permissions. To avoid this, we should ensure that the artifact file permissions start as 0666 (rw-rw-rw-).