Update access-om3 package to use external-parallelio variant needed with slack 0.20.1

[Quartzer2]

Moving to slack 0.20.1, access-om3 compile now gives

RUN spack -d install --only dependencies access-om3%intel@2021.1.2:
RuntimeError: variant "parallelio" not found in package "esmf" [required from package "access-om3"]

Need to update parallelio variant to external-parallelio