Lock firtool version to chisel-circt version

This is a clone of the issue reported by @oharboe here: https://github.com/sifive/chisel-circt/issues/21

tl;dr: We should have a mechanism (likely a separate repository) that sets the `firtool` version and grabs it via a special JAR file that contains host binaries of `firtool`.

Start by reviewing the linked chisel-circt issue for the original context. Define how the firtool version will be set and how a JAR containing host binaries will be obtained; the work is done when the project can reliably use the locked firtool version.

Written by the indexing model from the issue text.