Update Go to 1.20.12

Update the Omnibus builder image to a version shipping Go 1.20.12.

https://go.dev/doc/devel/release#go1.20:

go1.20.12 (released 2023-12-05) includes security fixes to the go command, and the net/http and path/filepath packages, as well as bug fixes to the compiler and the go command. See the Go 1.20.12 milestone on our issue tracker for details.