edsp: prevent it from trying to write a cache

EDSP(-like) protocols are one-shot processes working on data which
exists only as long as they run (as they get feed via a pipe), so trying
to write a cache for it is pretty pointless, especially as it will
usually fail as the cache files tend to be owned by root, but the
process is run as a unpriviledged user (either _apt if called by root,
otherwise the user of the caller).

So this was in fact only observeable with our testcases which run as
non-root and the worst which happens is that a valid cache is overridden
with an invalid one which the next run will detect and not use.

Git-Dch: Ignore

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