[Experiment] Reenable CI build cache for tests (#6390)

Let's see, if we can use `cargo-cache` again for the tests after #6389
reduced the number of test binaries to build that are quite large due as
they statically link copies of the same engine.
This might be one of the reasons why the tests on windows exceeded the
allotted disk space.

.github/workflows/ci.yml

@@ -82,11 +82,9 @@ jobs:
         #   toolchain: ${{ matrix.rust }}
         #   override: true
 
-      # Temporarily disabled; the cache was getting huge (2.6GB compressed) on Windows and causing issues.
-      # TODO: investigate why the cache was so big
-      # - uses: Swatinem/rust-cache@v1
-      #   with:
-      #     key: ${{ matrix.style }}v3 # increment this to bust the cache if needed
+      - uses: Swatinem/rust-cache@v1
+        with:
+          key: ${{ matrix.style }}v3 # increment this to bust the cache if needed
 
       - name: Tests
         uses: actions-rs/cargo@v1