ci: single, workspace-aware rust-cache for build_tauri_app (#4550)

* ci(pull-request): cache the vendored tauri workspace crates

build_tauri_app rebuilt the whole tauri stack every run. The fork is wired via
[patch.crates-io] to path crates (packages/tauri, packages/tauri-plugins) plus
local src-tauri/plugins/*, all workspace members that Swatinem/rust-cache prunes
by default (cache-workspace-crates: false). Every crates.io plugin depending on
the patched `tauri` then rebuilt transitively, while unrelated deps stayed cached.

Set cache-workspace-crates: true so those sporadically-updated submodule crates
are cached, and bump the cache key (tauri-cargo -> tauri-cargo-ws) so the old
workspace-crate-less cache is invalidated and repopulated (rust-cache won't
re-save on a full key match). The first run after this is a full rebuild;
subsequent runs reuse the cached tauri stack.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* ci(pull-request): keep a single rust-cache for build_tauri_app

build_tauri_app ran two rust-cache actions: actions-rust-lang/setup-rust-toolchain's
built-in one (cache-workspace-crates: false) plus the explicit Swatinem/rust-cache.
They doubled cache storage and competed over the shared target/. Set cache: false on
setup-rust-toolchain so the explicit cache — the one configured with
cache-workspace-crates for the vendored tauri fork — is the only one.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/pull-request.yml

@@ -297,11 +297,26 @@ jobs:
         uses: actions-rust-lang/setup-rust-toolchain@46268bd060767258de96ed93c1251119784f2ab6 # v1
         with:
           toolchain: stable
+          # Disable this action's built-in rust-cache so the explicit
+          # Swatinem/rust-cache below is the single cache (it's the one
+          # configured with cache-workspace-crates for the vendored tauri fork).
+          cache: false
 
       - uses: Swatinem/rust-cache@e18b497796c12c097a38f9edb9d0641fb99eee32 # v2
         with:
-          key: tauri-cargo
+          # cache-workspace-crates caches the path/workspace crates too: the
+          # vendored tauri fork (packages/tauri, packages/tauri-plugins, wired
+          # via [patch.crates-io]) and the local src-tauri/plugins/*. These are
+          # workspace members that rust-cache prunes by default, so the whole
+          # tauri stack — plus every crates.io plugin that depends on the
+          # patched `tauri` — rebuilt on every run. They change only
+          # sporadically (pinned submodules), so caching them is a big win.
+          # The key is bumped (-ws) so the old workspace-crate-less cache is
+          # invalidated and the next run repopulates it with the workspace
+          # crates included.
+          key: tauri-cargo-ws
           cache-all-crates: 'true'
+          cache-workspace-crates: 'true'
 
       - name: Cache apt packages
         uses: actions/cache@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5