ci: update cargo-rbmt and adopt toolchain mgmt

[nyonson]

The CI job was using a floating version of the cargo-rbmt install action (e.g. setup-rbmt@master) and the tool is still way too unstable for that, causing CI to die. Bumping the version and pinning it here.

This new version of cargo-rbmt includes the toolchain management, so the justfile and CI job have been simplified. It also has the new spot for the API files which doesn't include the package name.

The new version of cargo-rbmt also includes the random subset of features to test feature, which I believe exposed an issue in the v2 code (addressed in the first commit).

[nyonson]

@tcharding just a heads up, need this to unblock CI here. On the plus side, the new cargo-rbmt random feature tests found a little bug.

[nyonson]

If we wanna get a little tricky in the action, could compress a bunch of the job defintions down:

Check:
  runs-on: ubuntu-latest
  strategy:
    fail-fast: false
    matrix:
      command: [lint, api, docs, docsrs, integration]
  steps:
    - uses: actions/checkout@v6
    - uses: rust-bitcoin/rust-bitcoin-maintainer-tools/.github/actions/setup-rbmt@18e7f1788abfc14f5fb76204a3deab43b03ba39c
    - name: "Run ${{ matrix.command }}"
      run: cargo rbmt ${{ matrix.command }}

EDIT: couldn't help it, gave it a shot.

[tcharding]

ACK 730ea44