fastqtransform: avoid always requiring MB
fastqtransform: avoid always requiring MB

I'm using fastqtransform to get duplex on reads from the 5' end of
R1/R2. These get translated to CELL_CB1-CB2 without a MB UMI. This fix
avoids always requiring MB in the regex.

vals/umis,roryk/umis,vals/umis,roryk/umis