Use coursier cache

Author: Baccata

.github/workflows/ci.yml

@@ -12,6 +12,9 @@ jobs:
     - name: Checkout
       uses: actions/checkout@v2
 
+    - name: Cache
+      uses: coursier/cache-action@v6
+
     - name: Setup Java
       uses: actions/setup-java@v2
       with:
@@ -25,5 +28,5 @@ jobs:
 
     - name: Run tests
       run: |
-        mill -k --disable-ticker __.checkFormat &&
-        mill -k --disable-ticker __.test
+        mill -j 0 -k --disable-ticker __.checkFormat &&
+        mill -j 0 -k --disable-ticker __.test