fluids: Change Amat assemble flag name

[jrwrigh]

Differentiate between whether we're telling the code to actively assemble the matrix or just that we want Amat to be a mat type that can be assembled. Flag is new enough that nobody has used it before, so no need to deprecate, just replace.

[jeremylt]

Note - CI config changed so this will need to be rebased on main (may find this with other PRs too)