Make sure that pdiGcDaemon is the right pid

Move its definition before a loop that might change `$!`

tests/functional/common/vars-and-functions.sh.in

@@ -163,11 +163,11 @@ startGcDaemon() {
         -s "$NIX_STORE_DIR" \
         > /dev/null 2>&1 \
             &
+    pidGcDaemon=$!
     for ((i = 0; i < 30; i++)); do
         if [[ -S $NIX_GC_SOCKET_PATH ]]; then break; fi
         sleep 1
     done
-    pidGcDaemon=$!
     trapFunctions[killGcDaemon]=killGcDaemon
 }