Idea: Have CI check dev-shell works for configuring circt+LLVM ?

[dtzSiFive]

Presently we just instantiate the dev shell environment to ensure it evaluates+builds and that any work is cached.

While the shell has no guarantees, it would be nice to have CI or at least a script to see if that's sufficient for configuring CIRCT.

Normally using a nix shell based on a derivation sorta guarantees this already (since same inputs as used for building w/nix), but since specifying shell separately (and with support for python, unified build, other goodies) ensuring this stays working would be nice, especially as/when/if automation is added.