Don't show Cargo.lock warning if Cargo.toml has been modified

[jyn514]

Rustbot currently posts a warning on any PR that modifies Cargo.lock. this is helpful in cases where Cargo.lock was modified accidentally, for example if cargo ran with an old version of a submodule checked out. However, many PRs change Cargo.lock intentionally and shouldn't get the warning. As a heuristic, we can use "did any Cargo.toml change": if so, it's very likely the change was intentional.

@compiler-errors says this may be tricky since triage bot currently doesn't support conditional pings on files.

cc @BoxyUwU, I think you added this warning originally.

Originally posted by @rustbot in #103993 (comment)

[ChrisDenton]

Also perhaps if Cargo.lock is the only file modified? This happens when cargo updating dependencies.