Add CUPID_RUN_SERIAL to env_postprocessing.xml

[mnlevy1981]

If CUPID_RUN_SERIAL is TRUE, add --serial flag when running diagnostics

Description of changes:

• Please add an explanation of what your changes do and why you'd like us to include them.

All PRs Checklist:

• Have you followed the guidelines in our Contributor's Guide?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you made sure that the pre-commit checks passed (#8 in Adding Notebooks Guide)?
• Have you successfully tested your changes locally?

[mnlevy1981]

It looks like cupid-timeseries accepts the --serial flag, and uses it to determine whether or not to distribute calls of call_ncrcat():

        if serial:
            for cmd in list_of_commands:
                call_ncrcat(cmd)
        else:  # if not serial
            # Now run the "ncrcat" subprocesses in parallel:
            with mp.Pool(processes=num_procs) as mpool:
                _ = mpool.map(call_ncrcat, list_of_commands)
            # End with

however, the click description is copied from run_diagnostics:

$ git grep click.*serial cupid/
cupid/run_diagnostics.py:@click.option("--serial", "-s", is_flag=True, help="Do not use LocalCluster objects")
cupid/run_timeseries.py:@click.option("--serial", "-s", is_flag=True, help="Do not use LocalCluster objects")

So I want to clean that up in this PR as well