test: optionally build postgrest with cabal in postgrest-loadtest

By default, postgrest-with-pgrst builds postgrest as a nix package,
which means that source changes cause a full rebuild. With this
change, running the loadtest as

PGRST_BUILD_CABAL=1 postgrest-loadtest

rebuilds directly using cabal, like postgrest-build. Note that
results between nix and cabal builds aren't necessarily comparable
due to differing build parameters.

default.nix

@@ -122,6 +122,9 @@ rec {
   cabalTools =
     pkgs.callPackage nix/tools/cabalTools.nix { inherit devCabalOptions postgrest; };
 
+  withTools =
+    pkgs.callPackage nix/tools/withTools.nix { inherit cabalTools devCabalOptions postgresqlVersions postgrest; };
+
   # Development tools.
   devTools =
     pkgs.callPackage nix/tools/devTools.nix { inherit tests style devCabalOptions hsie withTools; };
@@ -154,9 +157,6 @@ rec {
       inherit (pkgs.haskell.packages."${compiler}") hpc-codecov;
       inherit (pkgs.haskell.packages."${compiler}") weeder;
     };
-
-  withTools =
-    pkgs.callPackage nix/tools/withTools.nix { inherit devCabalOptions postgresqlVersions postgrest; };
 } // pkgs.lib.optionalAttrs pkgs.stdenv.isLinux rec {
   # Static executable.
   inherit postgrestStatic;