postgresql: drop build-time dependency on GHC

This replaces `writeShellApplication` with `writeShellScriptBin` that
doesn't perform a shellcheck. This makes it way easier to build
postgresql on staging since GHC is super slow to build, even with pretty
powerful machines.

Also Haskell updates are currently merged straight into master which
means that postgresql and all reverse dependencies require a rebuild on
master then[1].

[1] NixOS#354270 (comment)

pkgs/servers/sql/postgresql/generic.nix

@@ -7,7 +7,7 @@ let
       , pkg-config, libxml2, tzdata, libkrb5, substituteAll, darwin
       , linux-pam, bison, flex, perl, docbook_xml_dtd_45, docbook-xsl-nons, libxslt
 
-      , removeReferencesTo, writeShellApplication
+      , removeReferencesTo, writeShellScriptBin
 
       , systemdSupport ? lib.meta.availableOn stdenv.hostPlatform systemdLibs
       , gssSupport ? with stdenv.hostPlatform; !isWindows && !isStatic
@@ -53,10 +53,7 @@ let
       else
         stdenv;
 
-    pg_config = writeShellApplication {
-      name = "pg_config";
-      text = builtins.readFile ./pg_config.sh;
-    };
+    pg_config = writeShellScriptBin "pg_config" (builtins.readFile ./pg_config.sh);
   in stdenv'.mkDerivation (finalAttrs: {
     inherit version;
     pname = pname + lib.optionalString jitSupport "-jit";