fix: add a dependency on current build run in Source_tree.root

[jchavarri]

Fixes #5549.

Adds a dependency on current build run in Source_tree.root. The fix was found by bisecting until I found the place where the parallelism in dune watch regressed (more details on original issue conversation). Hopefully the fix is correct, and someone with more knowledge can explain what the issue was, and why this fixes it 😅 Debugging and understanding memoization issues is not straightforward to me.

• The dependency on current build run was originally added in Re-implement File_tree on top of memoization #2706.
• Then removed in swallow stdout on success #4422, where dune watch parallelism regressed.

[jchavarri]

Hm, one realization while testing this branch is that incremental builds in watch mode that would be almost instant before are way slower in the branch because dune has to re-create rules, etc. So the parallelization works again, but at the cost of having to recalculate a lot of things that were already calculated.