Supply scratch directory through tar_resources()

[wlandau]

Prework

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Proposal

Currently, the scratch directory is always file.path(tar_config_get("store"), "scratch"). This is and #891 may contribute to #889. It would be better to supply scratch through tar_resources() (after #892 is implemented). This also makes it easier to make sure the internals of each target object know about a good scratch directory.

[wlandau]

Deciding against this because it gets really messy. Local targets can use the local data store for scratch files, and cloud targets can use files in tempdir(). All this is managed through path_scratch_dir() in R/utils_path.R.