The restic repository cache location defaults to ~/.cache/restic when
not overwritten either by the --cache-dir command line parameter or the
universal RESTIC_CACHE_DIR environment variable.
Currently, the --cache-dir variable is set to only some restic commands,
but, e.g., not to the unit's preStart command for the module's
initialize option. This results in two distinct cache locations, one at
~/.cache/restic for the initialize commands and one at the configured
--cache-dir location for the restic backup command.
By explicitly setting RESTIC_CACHE_DIR for the unit, only one cache at
the correct location will be used.
https://restic.readthedocs.io/en/v0.15.1/manual_rest.html#caching
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