change directory into installroot before launching arangod

[dothebart]

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As seen in https://github.com/arangodb/arangodb/issues/8120#issuecomment-516168335 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]