* 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.
Robert Vollmert