nixd/Server: fix dead semaphore in `Controller::onCompletion` (#241) #1436

Job	Run time
nix-build-develop-clang (none, release, true)	4m 9s
nix-build-develop-clang (none, release, false)	4m 6s
nix-build-develop-clang (none, debug, true)	5m 23s
nix-build-develop-clang (none, debug, false)	4m 53s
nix-build-develop-gcc (none, release, true)	4m 52s
nix-build-develop-gcc (none, release, false)	4m 32s
nix-build-develop-gcc (none, debug, true)	4m 26s
nix-build-develop-gcc (none, debug, false)	4m 34s
nix-build-develop-gcc (undefined, release, true)	10m 46s
nix-build-develop-gcc (undefined, release, false)	10m 34s
nix-build-develop-gcc (undefined, debug, true)	7m 1s
nix-build-develop-gcc (undefined, debug, false)	5m 38s
	1h 10m 54s

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