Separate SPD for ACCESS-OM2 biogeochemistry (BGC) build

[aidanheerdegen]

mom5 was updated to accept a type variant which allowed for the biogeochemistry version of MOM5 to be built by specifying type=ACCESS-OM-BGC.

ACCESS-OM2 can be built with the BGC version of MOM5 by specifying it in the spec, e.g. spack install mom5 type=ACCESS-OM-BGC.

However this causes some issues in the deployment GitHub workflows

ACCESS-NRI/ACCESS-OM2-BGC#2

To simplify this process it has been decided to make a dedicated access-om2-bgc spack package with the type variant hard-coded.

It looks like it should be possible to inherit from the access-om2 package to simplify this process

https://groups.google.com/g/spack/c/yeceTzEdq5w/m/lP1AXcuHCwAJ