Fix dune tests in ci

[serban-nicusor-toptal]

Don't run ocaml tests in isolated directories, cleanup at the end of the pipeline

Submission Checklist

• [ ] Run unit tests
• Documentation
  • [ ] If a user-facing facing change was made, the documentation PR is here:
  • [x] OR, no user-facing changes were made

Release notes

There seems to be an issue with dune running from a child directory inside stanc3. Before, we aimed for running each step with it's own source code isolated but since we were puttin stanc3 inside stanc3, example {path}/{stanc3 source code}/dune-tests/{stanc3 source code for this step} dune is running into an error

Entering directory '/tmp/stanc3'
Error: Too many opam files for package "stanc":
- dune-tests/stanc3/stanc.opam
- stanc.opam

This PR will execute the tests from the root of the working directory, therefore not running into this edge case.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[serban-nicusor-toptal]

Glad I could help!