Jenkins: Use pre-installed opam
Jenkins: Turn absolute symlinks created by ocamlbuild into relative

Since the workspace is bind-mounted inside a container, but Jenkins
runs outside the container, it can't handle absolute symlinks inside
the workspace.

'ocamlbuild' creates absolute symlinks though when building something.

This commit fixes this by using 'symlink -c' to turn the absolute links
inside the workspace root into relative ones.