ci: add riscv64 to release targets

[gounthar]

Adds riscv64gc-unknown-linux-gnu to the cargo-dist targets in dist-workspace.toml, with ubuntu-24.04-riscv as the custom runner.

The release workflow reads targets dynamically via dist plan, so no workflow changes needed.

RISE runners are provided by the RISE Project (https://riseproject.dev/), free for open source. To enable: install https://github.com/apps/rise-risc-v-runners on this repository. Without it, the riscv64 job stays queued with no runner.

Build validated on native riscv64: https://github.com/gounthar/atuin/actions/runs/23430141128

Closes #3046

[greptile-apps]

Greptile Summary

Adds riscv64gc-unknown-linux-gnu to cargo-dist release targets, using the RISE Project's ubuntu-24.04-riscv runner.

• The change itself is minimal and correct — target added to both targets and [dist.github-custom-runners].
• Key prerequisite: The RISE RISC-V Runners GitHub App must be installed on atuinsh/atuin before merging. Without it, release jobs for this target will queue indefinitely and block releases.
• No musl variant is added for riscv64, which is expected (Rust doesn't have a stable musl target for riscv64gc).

Important Files Changed

Filename	Overview
dist-workspace.toml	Adds riscv64gc-unknown-linux-gnu target with ubuntu-24.04-riscv custom runner. The runner depends on an external GitHub App (RISE Project) that must be installed separately — if it isn't, release jobs will queue indefinitely, blocking releases.

_{Reviews (1): Last reviewed commit: "ci: add riscv64gc-unknown-linux-gnu to c..." | Re-trigger Greptile}

[greptile-apps]

Blocking risk if RISE app isn't installed

If the RISE RISC-V Runners app is not installed on atuinsh/atuin, any release triggered will hang indefinitely waiting for the ubuntu-24.04-riscv runner — potentially blocking the entire release workflow.

Has the app been installed on this repository before merging?