Install completions during brew install

[kindermax]

https://github.com/grafana/gcx/blob/4d7779224224d1c4b282f38e868cb8ecfcebe1f3/docs/plans/2026-04-22-homebrew-formula-implementation.md#L4

https://github.com/grafana/gcx/blob/4d7779224224d1c4b282f38e868cb8ecfcebe1f3/.github/homebrew/gcx.rb.tmpl