postgresql: add PYTHONPATH via wrapProgram

Currently `PYTHONPATH` is not set when building PostgreSQL. This results
in Python modules not being available within `pl/python3u`.

This commit adds `PYTHONPATH` of the supplied `python3` via `wrapProgram`,
which is especially useful when supplying a custom Python env.

pkgs/servers/sql/postgresql/generic.nix

@@ -380,10 +380,14 @@ let
             --replace-fail '-bundle_loader $(bindir)/postgres' "-bundle_loader $out/bin/postgres"
         '';
 
-      postFixup = lib.optionalString stdenv'.hostPlatform.isGnu ''
-        # initdb needs access to "locale" command from glibc.
-        wrapProgram $out/bin/initdb --prefix PATH ":" ${glibc.bin}/bin
-      '';
+      postFixup =
+        lib.optionalString stdenv'.hostPlatform.isGnu ''
+          # initdb needs access to "locale" command from glibc.
+          wrapProgram $out/bin/initdb --prefix PATH ":" ${glibc.bin}/bin
+        ''
+        + lib.optionalString pythonSupport ''
+          wrapProgram "$out/bin/postgres" --set PYTHONPATH "${python3}/${python3.sitePackages}"
+        '';
 
       # Running tests as "install check" to work around SIP issue on macOS:
       # https://www.postgresql.org/message-id/flat/4D8E1BC5-BBCF-4B19-8226-359201EA8305%40gmail.com