So we can do `PGRST_DB_ANON_ROLE="" postgrest-run` since empty string has
meaning. argbash conflates both unset and empty string so this requires
a workaround.
* replaces v2-build -> v2-run (which should be v2-exec)
with one-shot v2-run
* fixes empty postgrest.prof file by sending SIGINT to the profiled
postgrest process to trigger writing it explicitly
```
$ postgrest-gen-secret
uMd97XSQzNkA1CWhMZ7u88Pj0RNyhrpo
$ postgrest-gen-jwt postgrest_test_author
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJyb2xlIjoicG9zdGdyZXN0X3Rlc3RfYXV0aG9yIn0.Xod-F15qsGL0WhdOCr2j3DdKuTw9QJERVgoFD3vGaWA
```
Also modifies postgrest-run to include a default PGRST_JWT_SECRET for
quicker manual tests.
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.
steve-chavez