Add riscv64 to release binaries

The current `.goreleaser.yml` builds Linux binaries for amd64, arm, arm64, and 386 but not for riscv64.

Since the project uses `CGO_ENABLED=0` and Go 1.22+, adding riscv64 should be a one-line change to the `goarch` lists in `.goreleaser.yml`:

```yaml
builds:
- id: binary
goarch:
- amd64
- arm
- arm64
- 386
- riscv64
```

(Same addition under the `snap` build id.)

I have built lazydocker from source on a native riscv64 machine (SpacemiT K1, Debian sid) and it runs without issues. Go riscv64 support has been stable since Go 1.16 and GoReleaser handles the architecture natively, so no other config changes are needed.

There is a growing number of RISC-V single-board computers and servers running Linux (Milk-V, StarFive, SpacemiT, Sipeed), and users on those boards would benefit from pre-built binaries rather than compiling from source.

Successful native riscv64 build and test run: https://github.com/gounthar/lazydocker/actions/runs/23535623437

Happy to open a PR if this seems reasonable.