Download the go binary directly

Rather than relying on the one included in the apk repository

This is a similar approach to what we use in the omnibus builder images.

The addition is that I add the symlink to alpine's libc library. It looks like fully compiling from source also requires doing this, or patching the sourcecode. https://git.alpinelinux.org/cgit/aports/tree/community/go/set-external-linker.patch?h=3.7-stable