only grab the header line from the first input VCF for final variants VCF.
only grab the header line from the first input VCF for final variants VCF.

EichlerLab/pacbio_variant_caller,EichlerLab/pacbio_variant_caller,EichlerLab/pacbio_variant_caller,EichlerLab/pacbio_variant_caller,EichlerLab/pacbio_variant_caller