[fx][script] Fix run-host-tests to point at top-level copy_host_tests gn rule

Change-Id: Iee37baf43c3bf4063324555b6a336ae50caadb52
diff --git a/scripts/devshell/run-host-tests b/scripts/devshell/run-host-tests
index e314d3c..9cb6d0b 100755
--- a/scripts/devshell/run-host-tests
+++ b/scripts/devshell/run-host-tests
@@ -44,7 +44,7 @@
     fx-command-run build-zircon "-v"
   else
     host_test_dir="${FUCHSIA_BUILD_DIR}/host_tests"
-    fx-command-run build build/gn:copy_host_tests
+    fx-command-run build :copy_host_tests
   fi
 
   runtests_cmd=("${ZIRCON_TOOLS_DIR}/runtests" "${ARGS[@]}")