Use the mathlib4 cache in CI build

The project build is now slower because we use mathllib4 in the project.
This commit adds a command to the build workflow to fetch the cached
build artefacts for mathlib (and other standard libraries).

This may speed up the CI build.

.github/workflows/build.yaml

@@ -30,6 +30,9 @@ jobs:
 
       - uses: actions/checkout@v4
 
+      - name: fetch mathlib cache
+        run: lake exe cache get
+
       - name: build project
         run: just build
 
@@ -63,6 +66,9 @@ jobs:
 
       - uses: actions/checkout@v4
 
+      - name: fetch mathlib cache
+        run: lake exe cache get
+
       - name: build project
         run: just build