[bazel] Disable WORKSPACE system

Prevents this from being re-enabled by accident.

Signed-off-by: James Wainwright <james.wainwright@lowrisc.org>

.bazelrc

@@ -2,8 +2,9 @@
 # Licensed under the Apache License, Version 2.0, see LICENSE for details.
 # SPDX-License-Identifier: Apache-2.0
 
-# Enable bzlmod using both `MODULE.bazel` and `WORKSPACE.bzlmod`.
+# Enable bzlmod with `MODULE.bazel` and disable the `WORKSPACE` system.
 common --enable_bzlmod
+common --enable_workspace=no
 
 # https://docs.opentitan.org/doc/rm/c_cpp_coding_style/#cxx-version specifies
 build --action_env=BAZEL_CXXOPTS="-std=gnu++14"