Allow fetching go binaries from a mirrored location

[nathanmayall]

Describe the Enhancement

I'd like to be able to host my own golang binary mirror and it would be nice to point the buildpack at that instead of being hardcoded to go.dev/dl/{version}

Possible Solution

An environment variable such as BP_GOLANG_BINARY_MIRROR or something along those lines

Motivation

Resiliency so I can self host my own golang distribution mirror and reduces my internet bandwidth.