E.g. replace `fs.Dir + filePath` with `path.Join(fs.Dir, filePath)`
The fs.Dir is guaranteed to end with slash - see Init() functions.
The filePath may start with slash. If it starts with slash, then `fs.Dir + filePath` constructs
an incorrect path with double slashes.
path.Join() properly substitutes duplicate slashes with a single slash in this case.
While at it, also substitute incorrect usage of filepath.Join() with path.Join()
for constructing paths to object storage systems, which expect forward slashes in paths.
filepath.Join() substittues forward slashes with backslashes on Windows, so this may break
creating or managing backups from Windows.
This is a follow-up for 0399367be602b577baf6a872ca81bf0f99ba401b
Updates https://github.com/VictoriaMetrics/VictoriaMetrics-enterprise/pull/719
