CI: trigger Rust job on rust-toolchain.toml changes

The `rust` paths-filter only watched `apps/desktop/src-tauri/**`, so
edits to `apps/desktop/rust-toolchain.toml` skipped the Rust CI jobs
entirely. The toolchain file shapes every Rust build on every runner;
not running the Rust suite when it changes is exactly backwards.
Add the toolchain file to the rust filter so PRs that touch it
actually exercise the jobs they affect.

Author: vdavid

.github/workflows/ci.yml

@@ -37,6 +37,7 @@ jobs:
           filters: |
             rust:
               - 'apps/desktop/src-tauri/**'
+              - 'apps/desktop/rust-toolchain.toml'
             svelte:
               - 'apps/desktop/src/**'
               - 'apps/desktop/static/**'