Add riscv64 to GoReleaser build matrix

Go projects cross-compile for riscv64 with no source changes. Since the goreleaser-full.yaml in this repo is shared across Charmbracelet tools (gum, bubbletea, vhs, etc.), adding riscv64 here would enable it for all projects at once.

Validated gum on native riscv64 hardware (BananaPi F3, SpacemiT K1, rv64gc) and on RISE runner.

The change would be adding `riscv64` to the `goarch` list in the builds section. Docker images would need a new template for linux/riscv64 as well.

I can submit a PR if useful.

Start in goreleaser-full.yaml, inspect the builds section and the Docker image templates. Add riscv64 to the Go architecture matrix and provide the linux/riscv64 image template, then validate that the shared configuration enables riscv64 builds and images for tools such as gum.

Written by the indexing model from the issue text.