Run tests of gixsql in CI with the new preprocessor

[Halbaroth]

This PR adds the tests of gixsql (https://github.com/OcamlPro/gixsql) in the CI of Superbol.
As you can check on my fork (Halbaroth#1), only 43 out of 69 tests succeed.

How it works:

• Nix creates an appropriate environment to compile gixsql and run its tests.
• A script run_test.sh located in the gixsql repository set up a complete test environment based on environment variables from a nix shell. This environment includes a PostgreSQL server that will run on localhost:6666.
• A wrapper of this script is present in the folder scripts/ of this repository. As run_test.sh fails if it isn't into an appropriate nix shell, this wrapper disables this script if nix is not on your computer.
• An action is called in the github workflow to install Nix.

Several caveats apply to the approach of this PR:

• I import the project gixsql.
• I install the latest release of GnuCOBOL in the Nix unstable channel, which means we do not test the preprocessor with the current development version of GnuCOBOL.
• If you have another PostgreSQL server on your machine which listen to localhost:6666, you will got an error with make test.

Currently, I active the verbosity mode of gixsql test runner, so the make test command produces a huge output. I plan to fix this soon.

This PR is rebased on #370.

[CLAassistant]

All committers have signed the CLA.

[nberth]

Like #375, this PR shall remain a draft until #370 is merged.