Use GOTOOLCHAIN to run older versions of the Go toolchain

I've been working through Mastering Distributed Tracing to learn more about (you guessed it) tracing. The book is from 2019, and while I've been assured that its contents are still sufficiently up-to-date, the Go-based code examples are a bit dated; namely, they use dep for dependency management, rather than the new(er) built-in module system. dep seems like a fine tool that had its time, but…