Remove the NIX_GC_SOCKET_PATH environment variable

Not really needed since it's configurable from the config (and people
can always use `$NIX_CONFIG` if they really need to configure it from
the CLI)

src/libstore/globals.hh

@@ -126,7 +126,7 @@ public:
 
     Setting<std::string> gcSocketPath {
         this,
-        getEnv("NIX_GC_SOCKET_PATH").value_or("auto"),
+        "auto",
         "gc-socket-path",
         "Path to the socket used to communicate with an external GC."
     };

tests/gc-external-daemon/common.sh

@@ -1,5 +1,6 @@
 source ../common.sh
 
 enableFeatures "external-gc-daemon"
+echo "gc-socket-path = $NIX_GC_SOCKET_PATH" >> "$NIX_CONF_DIR"/nix.cong
 
 startGcDaemon