ci: speedup & reduce cache storage (twilight-rs#2145)

Twilight's CI cache regularly exceeds the 10GB storage limit provided
by GitHub, which evicts the oldest caches once the limit is reached. The
resulting cache misses increases our CI time and wastes GitHub's
resources.

By excluding the book from the doctests job, the CI no longer needs to
recompile Twilight due to the book's dependencies. This should also
reduce this job's cache size.

Cache sizes for the `Test` job:

old: 438770249B (~418MB)
new: 383544825B (~366MB)

.github/workflows/check.yml

@@ -173,7 +173,7 @@ jobs:
         run: cargo nextest --config-file ${{ github.workspace }}/nextest.toml run --profile ci --workspace --exclude book
 
       - name: Run doctests
-        run: cargo test --doc
+        run: cargo test --doc --workspace --exclude book
 
   feature-combinations:
     name: Feature combinations