Support buildbox-casd running as separate user

Jürg Billeterrequested to merge
juerg/casd-separate-user into master

This fixes issues when running buildbox-casd as a separate user. This is necessary to protect the local cache of buildbox-casd from corruption without the use of FUSE. This is in preparation for the buildbox-run sandboxing backend (#1177 (closed)).

Edited by Jürg Billeter

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