hydra-server: Use server_store_uri instead of store_uri, if set

The store_uri may contain parameters that cause Nix to fail when running under the hydra-server user (e.g. the signing key).

Created by  Eelco Dolstra  on October 18, 2017
XUKUVWR4UKEKKOFLFMIHKOHHKSAYN3SWIF33GHUXRDVOBWWHOMMQC
Change contents