Add CI check to attempt building against minimum rust version

[Lyude]

It occurred to me that we should probably actually be verifying the minimum rust version in our build file, so let's add something for this to our CI

[theHamsta]

We could probably do this most easily by using the official Rust containers. I don't know whether Debian Buster has a new enough pango version. On Ubuntu, I had to build pango from source which is also not complicated.