Build hyprland-preview-share-picker with stable Rust

[scottjones]

Summary

Use stable Rust for hyprland-preview-share-picker 0.2.1. The recipe currently forces nightly during both dependency fetching and compilation, which selects an unnecessary nightly toolchain when Cargo is provided by rustup. The source builds with stable Rust; this is also the selection already carried in the Omarchy Mac package builder.

Change both RUSTUP_TOOLCHAIN assignments to stable and bump pkgrel to 2. The application sources, dependencies, locked Cargo build, and installed files are unchanged.

Validation

• Native aarch64 build passed through upstream's normal isolated builder with repository Rust 1.98.1. The build generated its configuration schema successfully.
• Inspected the ARM artifact: correct package version and architecture, executable AArch64 binary, and generated schema identical to the installed 0.2.1-1 package.
• Native x86_64 build and artifact inspection passed on a GitHub runner using the same recipe and upstream's builder: validation run. The job verified the package version, x86_64 executable, and generated schema.
• All seven commands from upstream's self-tests CI job passed locally; shell syntax and whitespace checks passed.

Interactive screen sharing was not retested during this packaging change.