[Debuginfod] Fix debuginfod unit test when $HOME is not writable.

The directory returned from `cache_directory` may not be writable if the unit tests are run with limited permissions (e.g. if $HOME is not writable).
This fixes a unit test of the debuginfod client to set the cache path to the system temporary directory to avoid this issue.

Reviewed By: estewart08

Differential Revision: https://reviews.llvm.org/D115813