integrations: allow multiarch image builds

[hugoShaka]

Part of #22732

This PR does the following things:

• Disable cgo in the operator and kube-agent-updater because it is not required
• Remove the bits about the arch-specific C compilator because it is not needed anymore without cgo
• Remove the Makefile logic which was choosing the right C compiler
• Add a Makefile target returning the current buildbox image

The goal behind this PR is to be able to build integrations images using the new oci-image-build workflow in teleport.e. This will provide the following advantages:

• add arm64 support for the operator
• build multiarch images
• provide signed images with our public key
• provide -debug variants of the images
• reuse the existing build and release logic from the -distroless images

The related e PR is here: https://github.com/gravitational/teleport.e/pull/1035