Subtree Update Automation Fixes

[thanhnguyen-aws]

Improvements to the workflow to update the subtree:

• Check for existing PRs before creating new ones
• Update the Kani version automatically
• Remove erroneous additional submodules
• Typo in toolchain file

Resolves#249

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[carolynzech]

Thanks for the quick PR. Are you also planning to fix the first item in the issue? We shouldn't mark this PR as resolving the issue until it fixes both.
(If you'd prefer to split #249 into two sub-issues, that's fine too).

[carolynzech]

Could you also incorporate these changes https://github.com/carolynzech/rust/tree/fix-subtree-bugs in this PR? (It skips the PR creations if they already exist, avoiding situations where we force push over commits that people have made to those PR branches since).

[carolynzech]

It'd also be nice to automate updating the Kani version in tool_config/ by finding the last commit to update rust-toolchain.toml, but that's not as important to get to in this PR (just an improvement, not a bug).

[tautschnig]

It'd also be nice to automate updating the Kani version in tool_config/ by finding the last commit to update rust-toolchain.toml, but that's not as important to get to in this PR (just an improvement, not a bug).

Oh, we could actually easily get the Kani commit id from the first step in the automation, which works in the Kani repository anyway.

[carolynzech]

Can you update the PR description and title to document the changes you made?

[carolynzech]

Can you please make these changes, then I'll approve? Perhaps @tautschnig or @qinheping could review as well.
I have not tested these suggested changes locally; please run it on your fork to make sure I'm right :)

[thanhnguyen-aws]

I ran this fixed Subtree Update on my fork and see that:

1. It updated Kani to the correct (most updated) version.
   (Merge subtree update for toolchain nightly-2025-02-21 thanhnguyen-aws/verify-rust-std#11)
2. When there exists similar PR, it does not create more PRs (and indeed terminated after finding the PR).
   (https://github.com/thanhnguyen-aws/verify-rust-std/actions/runs/13639811389/job/38126890075)