nix: more explicit logging in load test

* Write vegeta output to file explicitly.
  The previous version using `tee` captured the standard
  output of the `withPgrst` helper, too, causing weird
  errors when those had unexpected output.

* Log and fail the postgrest build explicitly.
  In situations such as changing nix dependencies, the
  `cabal build` run could fail, but this wasn't visible
  in CI.

With these changes, the load test now fails with a nice
error message when the build fails.

nix/tools/loadtest.nix

@@ -60,9 +60,8 @@ let
         # shellcheck disable=SC2145
         ${withTools.withPg} --fixtures "$_arg_testdir"/fixtures.sql \
         ${withTools.withPgrst} \
-        sh -c "cd \"$_arg_testdir\" && ${runner} -targets targets.http \"''${_arg_leftovers[@]}\"" \
-          | tee "$_arg_output" \
-          | ${vegeta}/bin/vegeta report -type=text
+        sh -c "cd \"$_arg_testdir\" && ${runner} -targets targets.http -output \"$_arg_output\" \"''${_arg_leftovers[@]}\""
+        ${vegeta}/bin/vegeta report -type=text "$_arg_output"
       '';
 
   loadtestAgainst =