fix test for availablity of mamba in container building

        if conda_version is not None:

            specs.append(f"conda={conda_version}")

        if mamba_version is not None:

            specs.append(f"mamba={mamba_version}")

            if mamba_version == "" and not specs:

                # If nothing but mamba without a specific version is requested,

                # then only run conda install if mamba is not already installed.

                mamba_test = "[ '[]' = \"$( conda list --json --full-name mamba )\" ]"

            specs.append(f"mamba={mamba_version}")

        conda_install = f"""conda install {verbose} --yes {" ".join(f"'{spec}'" for spec in specs)}"""

        involucro_args.extend(["-set", f"PREINSTALL=if {mamba_test} ; then {conda_install} ; fi"])