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

@@ -96,7 +96,7 @@ 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.sh

@@ -3,6 +3,8 @@ source common.sh
 sed -i 's/experimental-features .*/& external-gc-daemon/' "$NIX_CONF_DIR"/nix.conf
 
 export NIX_GC_SOCKET_PATH=$TEST_ROOT/gc.socket
+echo "gc-socket-path = $NIX_GC_SOCKET_PATH"
+
 startGcDaemon() {
     # Start the daemon, wait for the socket to appear.  !!!
     # ‘nix-daemon’ should have an option to fork into the background.