This ensures that switching between different substitute servers doesn't lead
to a polluted narinfo cache.
* guix/scripts/substitute.scm (narinfo-cache-file): Add 'cache-url'
parameter. Add the base32 of CACHE-URL as a sub-directory under
%NARINFO-CACHE-DIRECTORY. Update callers.
(cached-narinfo): Likewise. Call 'mkdir-p' on the dirname of the cache
file. Update callers.
(remove-expired-cached-narinfos): Add 'directory' parameter and use it
instead of %NARINFO-CACHE-DIRECTORY.
(narinfo-cache-directories): New procedure.
(maybe-remove-expired-cached-narinfo): Call 'remove-expired-cached-narinfos'
for each item returned by 'narinfo-cache-directories'.
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