rfc(engine): sandbox only when needed rather than throwing an error

[Alizter]

Problem

When a rule is configured to be sandboxed, however the action of the rule does not need sandboxing, we raise the following error message:

            User_error.raise
              ~loc
              [ Pp.text
                  "Rule dependencies are configured to require sandboxing, but the rule \
                   has no actions that could potentially require sandboxing."
              ]

This can appear in some rather cryptic locations such as:

• Inline_tests with empty partition  #8848
  • which led to a fix like Fix inline_tests action when partition list is empty #8849
• Add when action available in lockfiles #8443 (comment)

In this situation, it doesn't really make alot of sense that raise a user error for the following reasons:

1. Because incorrect sandbox configurations are generally out of the users control and can be internal to dune.
2. We have a better alternative solution which is to simply not sandbox.

Proposal 1

We should modify the behaviour in this case to simply conform to the sandboxing needs. If an action doesn't need to be sandboxed, then we should simply not sandbox it.

Proposal 2

In the aforementioned cases, the action in question was in fact the "empty actions" which is just (progn). Perhaps it is vacuously useful to sandbox in this case and we should just change it?

cc @rgrinberg @snowleopard

[Alizter]

This has appeared again when trying to fix #9026. The situation is that an empty (progn) can't be sandboxed, but that's ok and having it default to no sandboxing seems sensible.

[snowleopard]

I think we should just sandbox such actions. It doesn't seem particularly useful to insist on some sort of optimality. The current behaviour seems pretty annoying (imagine an action often flipping between requiring and not requiring sandboxing).

[Alizter]

A conservative fix for the issues I've been encountering would be to mark Prog [] as needing sandboxing. This would leave the error for other situations.

[snowleopard]

I don't understand why we need to be conservative. I'm inclined to just remove the error and do the sandboxing if the sandboxing configuration asks for it.