nix(loadtest): run all target branches almost at the same time

Instead of building, running, building, running, ... we now build all
executables once ahead of time and then run all loadtests right after
each other. This can sometimes reduce noise when load on the GHA runner
varies over time.

Since this requires us to move building into the loadtest-against
script, it also allows to go back to have the regular postgrest-loadtest
command default to building with cabal for faster local iteration.

default.nix

@@ -130,7 +130,7 @@ rec {
 
   # Development tools.
   devTools =
-    pkgs.callPackage nix/tools/devTools.nix { inherit tests style devCabalOptions hsie withTools; };
+    pkgs.callPackage nix/tools/devTools.nix { inherit tests style devCabalOptions hsie; };
 
   # Documentation tools.
   docs =