Basics for building PostgREST with Nix (#1489)

* Add GHC options used with stack to postgrest.cabal

* Add a Nix derivation for just the executable.

nix/nixpkgs-upgrade.nix

@@ -0,0 +1,41 @@
+{ curl
+, jq
+, writeShellScriptBin
+, nix
+}:
+# Utility script for pinning the latest unstable version of Nixpkgs.
+
+# Instead of pinning Nixpkgs based on the huge Git repository, we reference a
+# specific tarball that only contains the source of the revision that we want
+# to pin.
+let
+  name =
+    "nixpkgs-upgrade";
+
+  refUrl =
+    https://api.github.com/repos/nixos/nixpkgs/git/ref/heads/nixpkgs-unstable;
+
+  githubV3Header =
+    "Accept: application/vnd.github.v3+json";
+
+  tarballUrlBase =
+    https://github.com/nixos/nixpkgs/archive/;
+in
+writeShellScriptBin name
+  ''
+    set -euo pipefail
+
+    commitHash="$(${curl}/bin/curl "${refUrl}" -H "${githubV3Header}" | ${jq}/bin/jq -r .object.sha)"
+    tarballUrl="${tarballUrlBase}$commitHash.tar.gz"
+    tarballHash="$(${nix}/bin/nix-prefetch-url --unpack "$tarballUrl")"
+    currentDate="$(date --iso)"
+
+    cat << EOF
+    # Pinned version of Nixpkgs, generated with ${name}.
+    {
+      date = "$currentDate";
+      rev = "$commitHash";
+      tarballHash = "$tarballHash";
+    }
+    EOF
+  ''