GitLab

* Use ocaml-tree-sitter-core as a git submodule that's directly part of
the semgrep-core build.

* Don't build the executables defined in the git submodules. This leaves
only semgrep-core and spacegrep. Shaves off 3-4% from the build time and
keeps bin/ clean.

* Fix script

* Clean up

* Fix references to ocaml-tree-sitter in makefile

* Try to fix the CI error
Co-authored-by: pad <yoann.padioleau@gmail.com>