Fetch the binary and don't require a rust toolchain

[rsheeter]

Use published binary of compiler. That should mean we don't need a rust toolchain.

[rsheeter]

As far as I can tell releases/latest redirects but the download url doesn't support use of latest as a version so you end up with something like:

latest=$(curl -D - -s 'https://github.com/googlefonts/fontc/releases/latest' | grep location | awk -F/ '{sub(/[ \t\r\n]+/, "", $NF); print $NF}')
# GH redirects us to an objects.githubusercontent.com url so we have to follow redirects
curl -LJO "https://github.com/googlefonts/fontc/releases/download/${latest}/fontc-x86_64-unknown-linux-musl.tar.gz"