Loki 2.4.1 tries to `mkdir` a path relative to the working directory,
which is problematic when it is not set to a path that is writeable by
the loki user.
> level=error ts=2021-12-02T16:18:14.974316503Z caller=log.go:106 msg="error running loki" err="mkdir wal: permission denied [...]"
Fixes this by setting the working directory to lokis storage directory.
Signed-off-by: Martin Weinelt <martin.weinelt@man-da.de>
