[tools] Allow XAUTHORITY in tests (#21422)

This is required on Ubuntu 24.04 (when running in an interactive
session using Wayland, at least).

tools/bazel.rc

@@ -27,8 +27,10 @@ build --test_summary=terse
 # config_setting rules for proprietary software are provided in //tools.
 build --test_tag_filters=-gurobi,-mosek,-snopt
 
-# Inject DISPLAY into test runner environment for tests that use X.
+# Allow user environment variables relating to host graphics configuration to
+# flow through to tests that use X.
 build --test_env=DISPLAY
+build --test_env=XAUTHORITY
 
 # Location of the Gurobi license key file, typically named "gurobi.lic".
 # Setting this --test_env for all configurations is deliberate to improve