ci: Re-add llvm install to semver-checks run

[aborgna-q]

Alternative to #1908

I was a bit too eager when removing capnp in #1907.
https://github.com/CQCL/hugr/pull/1907/files#diff-8c3de78a6f07b22f56457fe701ccc74e767b3e22be85276c500eb1a81c01281eL46-L47

[hugrbot]

This PR contains breaking changes to the public Rust API.
Please deprecate the old API instead (if possible), or mark the PR with a ! to indicate a breaking change.

cargo-semver-checks summary

[aborgna-q]

Note that the workflow will continue to fail until merged, since it's running from a pull_request_target (so it uses main's definition instead of the local one)

[doug-q]

Nice