change directory into installroot before launching arangod

[dothebart]

As seen in arangodb/arangodb#8120 (comment) the arangodb starter should change the working directory to the install-root before launching arangod. In the rpm/deb case this would be / - in the tarball / windows installation this should be whatever directory there is in $0:

(new working directory)[/usr]/bin/arangodb[.exe]