Use `matrix` for golang-cross-compile

The [golang-cross-compile](https://github.com/buildkite/templates/tree/main/golang-cross-compile) template currently uses a plugin but I think we can achieve a similar outcome using the [`matrix` attribute](https://buildkite.com/docs/pipelines/command-step#matrix-attributes).

Start with the golang-cross-compile template and compare its current plugin-based setup with Buildkite's matrix attribute documentation. Confirm that the matrix approach provides the same cross-compilation outcome, then verify the template configuration and any available pipeline validation.

Written by the indexing model from the issue text.