Adding a cross-image temporary output directory

[NekkoDroid]

It would be nice to have an output directory that could be used to store intermediate files which do not need to persist after all the builds are done. This would need to be referrable by some specifier (for now %T).

This could be used as target OutputDirectory=%T for an initrd image, which only needs to exist until the "main" image has consumed it via Initrds=%T/initrd.cpio.zst. After all the builds are done this folder/its content would be deleted just like the workspace folder.