borg_client: use absolute path to borg in borg_cmd

Using plain "borg" resolves to /usr/local/bin/borg which is the wrapper
for our main backup host. This causes the offsite backup to be executed
with BORG_REPO set to the main backup destination.

While the above doesn't cause any issues with the backup script/service,
because borg invocations specify the backup destination as an argument,
it's not ideal and/or correct. Adjust borg_cmd to include the full path
of /usr/bin/borg, thus removing any ambiguity.