haskellPackages.ghcWithPackages: fix for GHC 8.10

[guibou]

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])

Then run /bin/ghci in the derivation and import PyF, it should import the module fine. I think I used PyF without any lose of generality.

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

Motivation for this change

Fixing GHC 8.10 ghcWithPackages which is currently not usable.

Things done

See above comments (Sorry, I did not realize that the template was after my commit message, so I submitted blindly)

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after) (0 changes)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[peti]

Please target the haskell-updates branch for these kind of changes.

[seanparsons]

@peti Is the intention that this fix wont make it into 20.03? As I'm still seeing it there with the latest that nix-prefetch-git gives me for that branch.

[peti]

@seanparsons, I think it would be perfectly fine to port that PR to release-20.03. I see no downsides.