Fix dev shell by including secp256k1 (#3710)

The dev shell should have the `libsecp256k1` library included, otherwise
`cabal build` fails, when building the `secp256k1-haskell` library:

```bash
[_61] rejecting: secp256k1-haskell-0.6.1 (conflict: pkg-config package libsecp256k1-any, not found in the pkg-config database)
```

flake.nix

@@ -85,6 +85,7 @@
                 supportedGhcVersions = [ "928" ];
               })
               final.z3
+              final.secp256k1
             ];
           all-cabal-hashes = final.fetchurl {
             url =