Let a use_cfgs entry pin its own tool

[martin-velay]

Part of lowRISC/dvplan#129.

Running a simulation regression under Xcelium and a formal regression under VC Formal in one invocation is a requirement for the bundle work, and it cannot be expressed today.

tool is the one key a config cannot set for itself once the command line gives it, by way of _CMDLINE_FIELDS in flow/hjson.py. Scalar merging is first-non-default-wins and raises on a genuine conflict, so a config setting tool: vcformal while importing OpenTitan's common_formal_cfg.hjson, which pins tool: jaspergold, fails to load with a conflicting-value error. The only way to select VC Formal is --tool vcformal on the command line, which applies to every config in the invocation.

Scope

• Let a use_cfgs entry carry a tool, seeded into that child's initial_values. Because arg_keys is computed from the keys present in initial_values, a seeded value wins over every imported file, which is exactly the mechanism --tool already uses. No changes needed in any OpenTitan or Mocha config.
• create_instance and _load_child_cfg in flow/base.py take the per-child overrides, and the factory closure in flow/factory.py applies them.
• --tool on the command line keeps overriding everything, since that is what it is for.
• Reject an override for a key other than tool for now, so this does not quietly become a general-purpose per-child config channel. That belongs with Improve the configuration loading #36.

Done when

• A primary config with one child pinned to xcelium and another pinned to vcformal loads, and each child's scratch path and tool config come from its own tool.
• A test covers the load, since the failure mode today is a RuntimeError from config merging and is easy to reintroduce.