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).

src/lib/Hydra/Helper/Nix.pm

@@ -496,7 +496,7 @@ sub restartBuilds($$) {
 
 sub getStoreUri {
     my $config = getHydraConfig();
-    return $config->{'store_uri'} // "auto";
+    return $config->{'server_store_uri'} // $config->{'store_uri'} // "auto";
 }