Plumb DONT_USE_ARGS to the script for automatic delay model generation

Automatic delay model generation's core is collecting data from synthesizing and timing individual XLS operators of various sizes, using Yosys and openSTA. A recent change requires that `DONT_USE_ARGS` be set properly for the target PDK -- it lists the cells that shouldn't be used by logic synthesis (e.g. clock gating cells). We need to set this properly in the `xls_delay_model_generation` bazel rules macro (https://github.com/google/xls/blob/main/xls/build_rules/xls_macros.bzl#L446).

For a temporary workaround, we can set `DONT_USE_ARGS` to `""`.