nix: fix static build

- the isExecutable patch was broken, remove it
- the ncurses fix was broken, we still need `enableStatic`
- the original error was caused by the new ghc-bignum
  package, which isn't a "proper" Haskell package; we filter
  this out explicitly now

nix/patches/static-haskell-nix-ncurses.patch

@@ -7,7 +7,7 @@ index 46d8066..a47f214 100644
                      "--enable-executable-static" # requires `useFixedCabal`
                      # `enableShared` seems to be required to avoid `recompile with -fPIC` errors on some packages.
 -                    "--extra-lib-dirs=${final.ncurses.override { enableStatic = true; enableShared = true; }}/lib"
-+                    "--extra-lib-dirs=${final.ncurses}/lib"
++                    "--extra-lib-dirs=${final.ncurses.override { enableStatic = true; }}/lib"
                    ]
                    # TODO Figure out why this and the below libffi are necessary.
                    #      `working` and `workingStackageExecutables` don't seem to need that,