Set `GOTOOLCHAIN=local` for in the `ENV` for all Go build stages
of the CNG.

This ensures that a project's checked in `toolchain` content from
`go.mod` will not override the version as configured for the CNG
pipelines itself. This greatly increases the likelihood we are
operating as intended, versus floating accidentally based on a
git checkout's content.

See also https://go.dev/doc/toolchain

Closes https://gitlab.com/gitlab-org/build/CNG/-/issues/2033

Changelog: changed