Consul 1.10 (PR #9792) introduced the ability to specify a prefix when
importing KV's. This however introduced a regression on Windows
systems which breaks `kv import`. The key name is joined with
specified`-prefix` using `filepath.Join()` which uses a forward slash
(/) to delimit values on Unix-based systems, and a backslash (\) to
delimit values on Windows – the latter of which is incompatible with
Consul KV paths.
This commit replaces filepath.Join() with path.Join() which uses a
forward slash as the delimiter, providing consistent key join behavior
across supported operating systems.
Fixes#10583
