cabal: fix upload to hackage (#1532)

When doing:

```
cabal upload dist-newstyle/sdist/postgrest-7.0.1.tar.gz
```

The following error is shown:

```
Error: Invalid package

'ghc-options: -Wall -Werror' makes the package very easy to break with
future GHC versions because new GHC versions often add new warnings.
Use just 'ghc-options: -Wall' instead. Alternatively, if you want to
use this, make it conditional based on a Cabal configuration flag
(with 'manual: True' and 'default: False') and enable that flag
during development.
```

Put -Werror in a cabal flag to work around this restriction.

default.nix

@@ -52,7 +52,7 @@ let
 
   # Base dynamic derivation for the PostgREST package.
   drv =
-    pkgs.haskellPackages.callCabal2nix name src {};
+    lib.enableCabalFlag (pkgs.haskellPackages.callCabal2nix name src {}) "FailOnWarn";
 
   # Static derivation for the PostgREST executable.
   drvStatic =