ci(pr-checks): cache cargo with rust-cache so the cache stops growing

actions/cache restored the previous target/ through restore-keys and
re-saved all of it under every new Cargo.lock hash, so each entry carried
every older build and the runner's cache grew without bound.
Swatinem/rust-cache strips the crate's own and stale artifacts before
saving and saves only on a miss.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

.gitea/workflows/pr-checks.yml

@@ -69,18 +69,17 @@ jobs:
           rustup target add x86_64-pc-windows-gnu
           cargo --version && cargo fmt --version && cargo clippy --version
 
-      # Keyed on Cargo.lock: dependency builds are reused until a dep actually
-      # changes. A cache miss only makes the run slower, never wrong.
-      - name: Cache cargo registry and build dir
-        uses: actions/cache@v4
+      # Dependency builds are reused until a dep actually changes; a cache miss
+      # only makes the run slower, never wrong. rust-cache rather than a plain
+      # actions/cache of target/: it strips this crate's own artifacts and
+      # anything stale before saving, and saves only on a miss. The plain cache
+      # restored the previous target/ and re-saved all of it under every new
+      # Cargo.lock, so each entry carried every older build and the runner's
+      # cache grew without bound.
+      - name: Cache cargo registry and dependency builds
+        uses: Swatinem/rust-cache@v2
         with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            sidecar/target
-          key: ${{ runner.os }}-cargo-${{ hashFiles('sidecar/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-
+          workspaces: sidecar -> target
 
       # Cheapest gate first — parses only, no compile, so a formatting slip fails
       # in seconds instead of after a full build.