Modernize CI cache and toolchain actions

Replace deprecated actions-rs/toolchain@v1 with dtolnay/rust-toolchain@stable
and the manual actions/cache@v3 setup with Swatinem/rust-cache@v2. The
purpose-built Rust cache action keys correctly on Cargo.lock + rustc version
(the old key only hashed Cargo.toml so cargo update wouldn't bust it), prunes
the parts of target/ that don't help on restore, and keeps build-script
outputs so the bitcoind/electrs binaries downloaded by corepc-node and
electrsd remain cached.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: benthecarman

.github/workflows/tests.yml

@@ -14,29 +14,17 @@ jobs:
     timeout-minutes: 60
     steps:
       - uses: actions/checkout@v4
-      - uses: actions-rs/toolchain@v1
+      - uses: dtolnay/rust-toolchain@stable
         with:
-          toolchain: stable
           components: clippy, rustfmt
-          override: true
-          profile: minimal
 
       - name: Install Protoc
         uses: arduino/setup-protoc@v3
         with:
           version: "30.2"
           repo-token: ${{ secrets.GITHUB_TOKEN }}
 
-      - uses: actions/cache@v3
-        with:
-          path: |
-            ~/.cargo/registry
-            ~/.cargo/git
-            target
-          key: cargo-${{ runner.os }}-rust-tests-${{ hashFiles('**/Cargo.toml') }}
-          restore-keys: |
-            cargo-${{ runner.os }}-rust-tests-
-            cargo-${{ runner.os }}-
+      - uses: Swatinem/rust-cache@v2.7.8
 
       - name: Check formatting
         run: cargo fmt --check