This update was generated by hackage2nix v2.15.1 from Hackage revision
commercialhaskell/all-cabal-hashes@fcda256.

* ghcHEAD: bump to 8.11.20200403

* ghcHead: reduce diff vs. 8.10.1

dontAddExtraLibs was removed by accident (IMO) in ea19a8e

* ghcHEAD: add ability to use system libffi

- enable nixpkgs' libffi
- minimise diffs against 8.10.1
- remove patching

* remove configure warning about --with-curses-includes

configure: WARNING: unrecognized options: --with-curses-includes

This also unbreaks `haskell-ci`.

- get the current Git head of haskell-tensorflow
- adjust dependencies

This closes #79441.

ghcWithPackages is using `ghc-pkg recache` to build its package
database. By doing so, it overrides the `package.cache[.lock]` files.

Details are unclear, but GHC 8.10 changed a bit the behavior.
Previously, it was unconditionally replacing the files by new ones. Now
it tries to open (for modification) the files. These files are symlinks
to another nix derivation, which is hence read-only.

This commit removes the files before running `ghc-pkg recache`, hence it
will just write the new files.

Tested with `haskellPackages.ghcWithPackages` (i.e. GHC 8.8) and
`haskell.packages.ghc8101.ghcWithPackages` (i.e GHC 8.10) with the
following nix file, at the root of the nixpkgs repository:

```
with import ./. {
  overlays = [
    (
      self: super: {
        haskellPackages = super.haskell.packages.ghc8101.override {
          overrides = selfh: superh: {
             th-lift-instances = super.haskell.lib.doJailbreak superh.th-lift-instances;
             th-expand-syns    = super.haskell.lib.doJailbreak superh.th-expand-syns;
             th-reify-many     = super.haskell.lib.doJailbreak superh.th-reify-many;
             th-orphans        = super.haskell.lib.doJailbreak superh.th-orphans;
             haskell-src-meta  = super.haskell.lib.doJailbreak superh.haskell-src-meta;
          };
        };
      }
  )
  ];
};
haskellPackages.ghcWithPackages(p:[p.PyF])
```

This will test with GHC 8.10. Comment out the `overlays` to test with
GHC 8.8.