The use of cabal freeze only works for install/configure. cabal fetch
does not honor the cabal.config.
Ref:
https://github.com/haskell/cabal/issues/4256
Change the build to still use the freeze file, however put the cabal
directory to $srcdir/cabal, do not set CABAL_CONFIG so that constraints
get shadowed if future packages are released, when installing libffi
constrain libffi to version 0.1 and extract constraints from the
cabal.config.
Additionally, for cabal freeze add switch to ignore the base ghc packages
as they're not relevant to cabal install/fetch/build.
Finally, add any libffi transitive depdendencies missing from the cabal.config
file for idris. For now only one applies. Also force constraints manually on
cabal install libffi as cabal.config wasn't getting picked up.
