🚦 Cache llvm-tools-preview

`cargo llvm-cov` automatically installs the `llvm-tools-preview` via
Rustup, but this build does _not_ get cached unlike other tools that
are installed. A consequence of this is that this makes the build
take longer.

This now explicitly requests it so that it can be cached for later
builds.

.github/workflows/build.yaml

@@ -171,6 +171,8 @@ jobs:
 
       - name: Install toolchain
         uses: ./.github/actions/rust-toolchain
+        with:
+          components: llvm-tools-preview
 
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov