Workflow improvements

[vchuravy]

I noticed two immediate workflow/CI issues:

1. mlir-jl-tblgen should probably have it's own CI

• Maybe we can use Preferences.jl to allow developers to override the Dialects dir=

2. Would be nice if I could run bindings/make.jl against a locally compiled version of mlir-jl-tblgen (but for all LLVM versions)

• This would allow us to bump it's version and see the changes to the wrappers immediatly.