This allows more flexible control over the working directory. Values for workingDir must always start
with a / and will then be relative to the repo root.
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.
