postgresql_14: fix build

Building postgresql_14 currently fails with this on master:

  error: derivation contains an illegal reference specifier 'man'

The odd thing is, that changing *anything* in the postInstall phase
seems to fix this, including just adding a single character as a
comment.

I have no clue what's happening here.

pkgs/servers/sql/postgresql/generic.nix

@@ -307,6 +307,7 @@ let
       postInstall =
         ''
           moveToOutput "bin/ecpg" "$dev"
+          # The following seems to fail without this comment (?)
           moveToOutput "lib/pgxs" "$dev"
 
           # Pretend pg_config is located in $out/bin to return correct paths, but