nominatim: fix issues with package

There are two issues that make the package unusable as is:
- The python dependencies are not taken into account, giving import
  errors on launch
- nominatim needs to be built with the same postgres version it will run
  on. It's a bit dirty to do an override postgresql_12 =
  postgresql_another_version, so this commit adds a `postgres` attribute
  to be more consistent

pkgs/servers/nominatim/default.nix

@@ -1,7 +1,8 @@
 { stdenv, lib, fetchFromGitHub, fetchurl
-, clang-tools, cmake, bzip2, zlib, expat, boost, git, pandoc, gzip
-, postgresql_12
-, python3, python3Packages, php
+, clang-tools, cmake, bzip2, zlib, expat, boost, git, pandoc
+# Nominatim needs to be built with the same postgres version it will target
+, postgresql
+, python3, php
 }:
 
 let
@@ -36,21 +37,19 @@ stdenv.mkDerivation rec {
     zlib
     expat
     boost
-    python3
+    (python3.withPackages (ps: with ps; [
+      pyyaml
+      python-dotenv
+      psycopg2
+      psutil
+      jinja2
+      pyicu
+      datrie
+    ]))
     # python3Packages.pylint  # We don't want to run pylint because the package could break on pylint bumps which is really annoying.
     # python3Packages.pytest  # disabled since I can't get it to run tests anyway
     # python3Packages.behave  # disabled since I can't get it to run tests anyway
-    postgresql_12
-  ];
-
-  propagatedBuildInputs = with python3Packages; [
-    pyyaml
-    python-dotenv
-    psycopg2
-    psutil
-    jinja2
-    pyicu
-    datrie
+    postgresql
   ];
 
   postPatch = ''

pkgs/top-level/all-packages.nix

@@ -1953,7 +1953,9 @@ with pkgs;
 
   node-glob = callPackage ../tools/misc/node-glob { };
 
-  nominatim = callPackage ../servers/nominatim { };
+  nominatim = callPackage ../servers/nominatim {
+    postgresql = postgresql_12;
+  };
 
   npm-check-updates = callPackage ../tools/package-management/npm-check-updates { };