CI improvements

- cache RTIM for the style checks
- disable fail-fast for main checks so that we can see whether the other jobs are green

src/tools/miri/.github/workflows/ci.yml

@@ -22,6 +22,7 @@ jobs:
       RUST_BACKTRACE: 1
       HOST_TARGET: ${{ matrix.host_target }}
     strategy:
+      fail-fast: false
       matrix:
         build: [linux64, macos, win32]
         include:
@@ -89,11 +90,46 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - name: Install required toolchain
-        # We need a toolchain that can actually build Miri, just a nightly won't do.
+
+      # This is exactly duplicated from above. GHA is pretty terrible when it comes
+      # to avoiding code duplication.
+
+      # Cache the global cargo directory, but NOT the local `target` directory which
+      # we cannot reuse anyway when the nightly changes (and it grows quite large
+      # over time).
+      - name: Add cache for cargo
+        id: cache
+        uses: actions/cache@v3
+        with:
+          path: |
+            # Taken from <https://doc.rust-lang.org/nightly/cargo/guide/cargo-home.html#caching-the-cargo-home-in-ci>.
+            ~/.cargo/bin
+            ~/.cargo/registry/index
+            ~/.cargo/registry/cache
+            ~/.cargo/git/db
+            # contains package information of crates installed via `cargo install`.
+            ~/.cargo/.crates.toml
+            ~/.cargo/.crates2.json
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-cargo
+
+      - name: Install rustup-toolchain-install-master
+        if: ${{ steps.cache-npm.outputs.cache-hit != 'true' }}
+        shell: bash
+        run: |
+          cargo install -f rustup-toolchain-install-master
+
+      - name: Install "master" toolchain
+        shell: bash
         run: |
-          cargo install rustup-toolchain-install-master # TODO: cache this?
           ./rustup-toolchain "" -c clippy
+
+      - name: Show Rust version
+        run: |
+          rustup show
+          rustc -Vv
+          cargo -V
+
       - name: rustfmt
         run: ./miri fmt --check
       - name: clippy