feat(cli): add RISC-V 64-bit pre-built binary support (#14685)

* feat(cli): add RISC-V 64-bit pre-built binary support

Add riscv64gc-unknown-linux-gnu target to the tauri-cli release workflow,
enabling pre-built binaries for RISC-V 64-bit Linux systems.

This eliminates the multi-hour QEMU compilation time that currently blocks
RISC-V adoption of Tauri apps. Native compilation on RISC-V hardware
takes ~63 minutes.

Changes:
- Add RISC-V entry to build matrix with self-hosted runner support
- Support custom `runs_on` field for matrix entries (falls back to `os`)
- Skip dtolnay/rust-toolchain and rust-cache for self-hosted runners
- Source ~/.cargo/env for self-hosted runners where Rust is pre-installed

Tested on:
- Hardware: Banana Pi F3 (RISC-V64, 16GB RAM)
- OS: Debian Trixie (required for WebKit2GTK RISC-V support)
- Build time: 1h 2m 28s
- Binary: ELF 64-bit RISC-V, 16MB stripped

* feat(cli): use cross for RISC-V cross-compilation

Switch from self-hosted runners to cross-rs for building RISC-V binaries.
This approach is simpler and doesn't require maintaining self-hosted infrastructure.

Local testing confirms cross builds a valid RISC-V binary in ~4 minutes.

* refactor(cli): address review feedback for RISC-V workflow

- Skip Rust toolchain and cache setup for cross builds (unnecessary)
- Pin cross version to 0.2.5 for reproducibility
- Fix Linux dependencies condition to match ubuntu-* variants