Do not substitute `@...@` for stage-specific values in cabal files
rts and ghc-prim now no longer have a *.cabal.in to set Cabal flag
defaults; instead manual choices are passed to configure in the usual
way.
The old way was fundamentally broken, because it meant we were baking
these Cabal files for a specific stage. Now we only do stage-agnostic
@...@ substitution in cabal files (the GHC version), and so all
stage-specific configuration is properly confined to _build and the
right stage dir.
Also include-ghc-prim is a flag that no longer exists for ghc-prim
(it was removed in 835d8ddb) so I got rid of it.
Progress towards #19174