nixd/Server/Controller: fix potential data race while reading config … #1394

Job	Run time
nix-build-develop-gcc (none, release, true)	5m 56s
nix-build-develop-gcc (none, release, false)	4m 49s
nix-build-develop-gcc (none, debug, true)	4m 11s
nix-build-develop-gcc (none, debug, false)	4m 28s
nix-build-develop-gcc (undefined, release, true)	13m 42s
nix-build-develop-gcc (undefined, release, false)	10m 10s
nix-build-develop-gcc (undefined, debug, true)	6m 27s
nix-build-develop-gcc (undefined, debug, false)	6m 21s
nix-build-develop-clang (none, release, true)	4m 10s
nix-build-develop-clang (none, release, false)	3m 57s
nix-build-develop-clang (none, debug, true)	3m 45s
nix-build-develop-clang (none, debug, false)	3m 40s
	1h 11m 36s

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